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Subaru · Engine · Moderate code

P0303 on Subaru: How to Diagnose & Fix

Cylinder 3 Misfire Detected

8 Subaru models · 11 documented issues · 9 critical · $60-$12,000 repair

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P0303 by Subaru model

  • 1999-2019Outback2
  • 1993-2011Impreza2
  • 2000-2009Legacy1
  • 2003-2006Baja1
  • 2015-2021WRX STI1
  • 2013-2016BRZ1
  • 2006-2014Tribeca2
  • 2004-2013Forester1
  • FAQ

P0303 on other makes

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  • Alfa Romeo
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  • Gmc
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P0303 by Subaru model

  • 1999-2019Outback2
  • 1993-2011Impreza2
  • 2000-2009Legacy1
  • 2003-2006Baja1
  • 2015-2021WRX STI1
  • 2013-2016BRZ1
  • 2006-2014Tribeca2
  • 2004-2013Forester1
  • FAQ

P0303 on Subaru by model: symptoms, diagnosis and fix

Community reported
4,521 owners
Link to this issueError Codes:P0171P0174P0300P0301P0302P0303P0304P0128

The 2.5L non-turbo engine is known for external head gasket leaks. Oil seeps past the gasket and can contaminate the coolant. Subaru extended warranties and improved gasket design in later years.

Common Symptoms

  • Oil leaking externally from engine
  • Coolant mixing with oil (milky appearance)
  • Overheating
  • Sweet smell from engine bay
  • Bubbles in coolant reservoir

How to Fix

Replace head gaskets with updated MLS (multi-layer steel) gaskets. Ensure the machine shop resurfaces cylinder heads to proper flatness. Replace timing belt, water pump, and thermostat while engine is apart. Use Subaru coolant conditioner to help prevent future failures. Budget $1,500-2,500 for a quality repair.

What you need to fix it

Only diagnosis- and fitment-reviewed repair parts are linked here.

  • Genuine Subaru 11044AA610 EJ25D Cylinder-Head Gasket

    head gasket for each bank on the 1999 2.5-liter branch after failure is confirmed Fitment/availability: 1999 Outback 2.5-liter EJ25D DOHC branch; quantity two, confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart…

    SubaruPartsDeal
  • Genuine Subaru 11044AA633 SOHC Cylinder-Head Gasket

    head gasket for each bank on the 2000-2009 naturally aspirated branch Fitment/availability: 2000-2009 Outback naturally aspirated 2.5-liter branch; quantity two, confirm VIN and exclude turbo Fitment/availability: ships in 1-3 business d…

    SubaruPartsDeal
  • Genuine Subaru 11044AA770 Cylinder-Head Gasket

    head gasket for each bank on the 2010 branch Fitment/availability: 2010 Outback naturally aspirated 2.5-liter EJ253 branch via supersession from 11044AA670; quantity two, confirm VIN Fitment/availability: ships in 1-3 business days with …

    SubaruPartsDeal
  • Genuine Subaru 13028AA072 EJ25D Timing Belt

    timing belt when removed, due or of unknown history on the 1999 branch Fitment/availability: 1999 Outback 2.5-liter EJ25D DOHC branch; confirm VIN Fitment/availability: ships in 1-2 business days with Add to Cart live when reviewed

    SubaruPartsDeal
  • Genuine Subaru 13028AA21B SOHC Timing Belt

    timing belt when removed, due or of unknown history on the 2000-2009 branch Fitment/availability: 2000-2009 Outback naturally aspirated 2.5-liter branch; confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart live …

    SubaruPartsDeal
  • Genuine Subaru 13028AA231 Timing Belt

    timing belt when removed, due or of unknown history on the 2010 branch Fitment/availability: 2010 Outback naturally aspirated 2.5-liter EJ253 branch; confirm VIN and production date Fitment/availability: ships in 1-3 business days with A…

    SubaruPartsDeal
  • Genuine Subaru 13033AA042 Hydraulic Timing-Belt Tensioner

    tensioner renewed with a due or unknown-history complete timing service Fitment/availability: 1999-2010 Outback 2.5-liter timing-belt branches in this card; confirm VIN and configuration Fitment/availability: ships in 1-3 business days w…

    SubaruPartsDeal
  • Genuine Subaru 21111AA110 Water Pump

    early water pump only when failed, leaking, rough, due or selected for unknown-history service Fitment/availability: 1999-2005 Outback 2.5-liter branch within this card; confirm VIN and required seal Fitment/availability: ships in 1-2 bu…

    SubaruPartsDeal
  • Genuine Subaru 21111AA280 Water Pump

    middle-year water pump when condition or service history warrants replacement Fitment/availability: 2006-2009 Outback naturally aspirated 2.5-liter branch; confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart liv…

    SubaruPartsDeal
  • Genuine Subaru 21111AA320 Water Pump

    2010 water pump when condition or service history warrants replacement Fitment/availability: 2010 Outback naturally aspirated 2.5-liter branch; confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart live when reviewed

    SubaruPartsDeal
  • Genuine Subaru 21210AA030 Thermostat and Gasket Assembly

    thermostat only when diagnosis or condition warrants replacement on the 2000-2009 branch Fitment/availability: 2000-2009 Outback 2.5-liter branch; the 1999 branch requires VIN-specific selection Fitment/availability: Add to Cart live whe…

    SubaruPartsDeal
  • Genuine Subaru 21210AA080 Thermostat and Gasket Assembly

    2010 thermostat only when diagnosis or condition warrants replacement Fitment/availability: 2010 Outback naturally aspirated 2.5-liter branch; confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart live when reviewed

    SubaruPartsDeal

As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.

Owner tips & cautions

  • TipDIY repairs can save significantly - dealer charges $1,500-2,500 but DIY costs are typically 50-70% less
  • TipUse Subaru coolant conditioner to help prevent future failures.
  • WarningThis is a high-severity issue - ignoring it can lead to costly repairs or safety concerns. Address it promptly.
  • TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
Typical repair cost:$1,500 - $2,500
Sources for this issue (4)
  • Head Gasket Failure in 2.2 liter models - Subaru Outback Forumsforum
  • Subaru Head Gasket Problems Explained - All Wheel Drive Automedia
  • Ej25 sohc 2000 ob head gasket - Ultimate Subaruforum
  • Subaru Head Gasket Problems Crucial Years - Automotive Body Caremedia

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Based on 4,521 owner reports

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Community Reported
High ConfidenceVerified4,521 owner reportsLast reported by owners Jan 2024Reviewed Feb 2026
Community reported
680 owners
Link to this issueError Codes:P0171P0174P0300P0301P0302P0303P0304P0128

While the FB25 has improved head gasket reliability compared to older EJ engines, some vehicles still experience head gasket failures. External oil leaks and coolant leaks can develop, particularly at higher mileage.

Common Symptoms

  • Oil leak near head gasket
  • External coolant leak
  • Overheating
  • Sweet coolant smell
  • Oil mixing with coolant (rare)

How to Fix

Monitor for coolant and oil leaks around head gasket area. Replace head gaskets if leaking. Use proper Subaru coolant. This is less common than on older EJ engines but still occurs.

Owner tips & cautions

  • TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
  • TipSearch Subaru Outback forums and owner groups for real-world experiences and DIY guides
  • TipGet multiple quotes from independent mechanics - dealer prices can be 2-3x higher for the same repair
Typical repair cost:$1,200 - $2,500
Sources for this issue (1)
  • Subaru-Forums.com thread discussing FB25 head gasket failuresforum

Research This Issue

Based on 680 owner reports

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Community Reported
Medium Confidence680 owner reportsLast reported by owners Feb 2023Reviewed Feb 2026
All Subaru Outback known issues →
Community reported
4,200 owners
Link to this issueError Codes:P0171P0174P0300P0301P0302P0303P0304P0128

The 2.5L EJ253 naturally-aspirated engine in 1999-2011 Imprezas is notorious for external head gasket failure. The composite head gaskets degrade over time, allowing coolant and oil to seep externally at the gasket mating surfaces. Unlike a typical blown head gasket that causes coolant-oil mixing, Subaru EJ25 head gaskets typically leak externally first. The boxer engine design means gaskets are at the bottom of the heads, making leaks drip downward and often go unnoticed. If left unaddressed, external leaks can progress to internal coolant-oil mixing and overheating.

Common Symptoms

  • Oil seepage visible on bottom of engine near head gasket area
  • Coolant level slowly dropping without visible external puddles
  • Sweet coolant smell from engine bay
  • Oil spots on garage floor near front of vehicle
  • Overheating in severe cases (internal gasket breach)
  • Bubbles in coolant overflow reservoir

How to Fix

Replace both head gaskets with updated MLS (multi-layer steel) gaskets (Subaru OEM 11044AA770 or Six Star MLS set). Resurface both cylinder heads to within 0.002 inch flatness spec. Replace timing belt, water pump (21111AA370), thermostat, and all seals while the engine is apart. Add Subaru Cooling System Conditioner (SOA635071) after repair. Budget 8-12 hours labor for the complete job.

What you need to fix it

Only diagnosis- and fitment-reviewed repair parts are linked here.

  • Genuine Subaru 11044AA633 Naturally Aspirated Head Gasket

    current OE MLS head gasket after the affected engine and failure are confirmed Fitment/availability: 1999-2011 naturally aspirated 2.5-liter Impreza EJ251/EJ253 applications; quantity two, confirm VIN and engine Fitment/availability: Add…

    Subaru Online Parts
  • Genuine Subaru 21111AA007 Water Pump

    water pump only when failed, contaminated or due during accessible timing service Fitment/availability: 1999-2005 naturally aspirated 2.5-liter Impreza branch within this issue; confirm VIN and engine Fitment/availability: ships in 1-3 b…

    SubaruPartsDeal
  • Genuine Subaru 21111AA280 Water Pump

    later-branch water pump only when failed, contaminated or due during accessible timing service Fitment/availability: 2006-2011 naturally aspirated 2.5-liter Impreza branch within this issue; confirm VIN and engine Fitment/availability: A…

    Subaru Parts Guys
  • Genuine Subaru 21200AA072 Thermostat Assembly

    thermostat when testing, age or contamination warrants replacement Fitment/availability: Use here for the confirmed 1999-2005 naturally aspirated Impreza branch; verify VIN and catalog split Fitment/availability: ships in 1-3 business da…

    SubaruPartsDeal
  • Genuine Subaru 21200AA170 Thermostat Assembly

    later-branch thermostat when testing, age or contamination warrants replacement Fitment/availability: 2006-2011 naturally aspirated 2.5-liter Impreza branch within this issue; confirm VIN and engine Fitment/availability: ships in 1-3 bus…

    SubaruPartsDeal

As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$1,500 - $2,800
Sources for this issue (4)
  • Subaru Head Gasket Problems Explained - All Wheel Drive Automedia
  • EJ251 Head Gasket Problems - RS25forum
  • Head Gasket Problems Explained - RS25forum
  • Head gasket problem fixed in 2001 2.5 RS - RS25forum

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Based on 4,200 owner reports

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Community Reported
High Confidence4,200 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026
Link to this issueError Codes:P0300P0301P0302P0303P0304

The SOHC EJ18 and EJ22 engines in the base 1990s Impreza use a camshaft-driven distributor. With age, the cap and rotor corrode and crack, and the distributor O-ring can weep oil into the cap. This causes crossfire and weak spark that shows up as rough idle, hesitation, and hard starting, classically worse in damp or rainy weather when moisture inside the cap promotes arcing. It is an inexpensive but frequently overlooked maintenance item on these high-mileage cars.

Common Symptoms

  • Rough idle and intermittent misfire
  • Hesitation or stumble under acceleration
  • Hard starting or stalling, worse in wet/humid weather
  • Check-engine light with misfire codes (OBD-II cars)

How to Fix

Replace the distributor cap, rotor, and spark plug wires (and plugs) with quality parts; on OBD-II cars this clears random/cylinder misfire codes. If oil is found inside the distributor, replace the distributor O-ring/seal. Keep ignition components dry and correctly seated.

Typical repair cost:$60 - $250
Sources for this issue (2)
  • EJ22 Distributor? - Ultimate Subaru Message Boardforum
  • EJ22 Random misfire when warmed - Ultimate Subaru Message Boardforum

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Community Reported
Medium ConfidenceVerifiedReviewed Jul 2026
All Subaru Impreza known issues →
Community reported
3,800 owners
Link to this issueError Codes:P0171P0174P0300P0301P0302P0303P0304P0128

The 2.5L EJ25 engine in 2000-2009 Legacys suffers from the same head gasket failure that plagues all EJ25-equipped Subarus. The composite gaskets degrade, causing external oil and coolant leaks at the head-to-block mating surfaces. The Legacy's higher-mileage highway use means many owners encounter this between 80,000-150,000 miles. Phase 1 engines (2000-2005, single overhead cam) are more prone to coolant leaks, while Phase 2 engines (2006-2009, DOHC) tend to leak oil externally first.

Common Symptoms

  • External oil seepage at bottom of cylinder heads
  • Coolant level gradually decreasing
  • Oil and coolant mixing (milky residue on oil cap)
  • Overheating at highway speeds
  • Sweet smell from engine compartment
  • White exhaust smoke in severe cases

How to Fix

Replace both head gaskets with updated MLS (multi-layer steel) gaskets. Use Six Star MLS gasket kit or Subaru OEM MLS (11044AA633 for SOHC, 11044AA770 for DOHC). Machine heads to 0.002-inch flatness. Replace timing belt, tensioner, idlers, water pump, thermostat, and valve cover gaskets while apart. Use Subaru Cooling System Conditioner (SOA635071). Total job is 10-14 hours labor.

What you need to fix it

Only diagnosis- and fitment-reviewed repair parts are linked here.

  • Genuine Subaru 11044AA633 SOHC Cylinder-Head Gasket

    OE head gasket for each bank after failure is confirmed on the naturally aspirated branch Fitment/availability: 2000-2009 Legacy naturally aspirated 2.5-liter SOHC branch; quantity two, confirm VIN and exclude turbo GT Fitment/availabili…

    SubaruPartsDeal
  • Genuine Subaru 11044AA642 Turbo Cylinder-Head Gasket

    OE head gasket for each bank on the early Legacy GT turbo branch Fitment/availability: 2005-2006 Legacy GT and GT Limited 2.5-liter turbo branch; quantity two, not for the naturally aspirated engine Fitment/availability: ships in 1-3 bus…

    SubaruPartsDeal
  • Genuine Subaru 11044AA770 Turbo Cylinder-Head Gasket

    OE head gasket for each bank on the later Legacy GT turbo branch Fitment/availability: 2007-2009 Legacy GT and GT Limited 2.5-liter turbo branch; quantity two, confirm VIN and production date Fitment/availability: direct Add to Cart prod…

    SubaruPartsDeal
  • Genuine Subaru 13028AA21B Naturally Aspirated Timing Belt

    timing belt when removed, due or of unknown history during the naturally aspirated repair Fitment/availability: 2000-2009 Legacy naturally aspirated 2.5-liter SOHC branch; confirm VIN Fitment/availability: ships in 1-3 business days with…

    SubaruPartsDeal
  • Genuine Subaru 13028AA240 Turbo Timing Belt

    timing belt when removed, due or of unknown history during the turbo repair Fitment/availability: 2005-2009 Legacy GT 2.5-liter turbo branch within this card; retailer catalogs the broader 2004-2012 turbo range, so confirm VIN Fitment/av…

    SubaruPartsDeal
  • Genuine Subaru 21111AA110 Naturally Aspirated Water Pump

    water pump when failed, leaking, rough, due or selected for an unknown-history full timing service Fitment/availability: 2000-2005 Legacy naturally aspirated 2.5-liter branch; confirm VIN and required gasket/seal Fitment/availability: sh…

    SubaruPartsDeal
  • Genuine Subaru 21111AA280 Naturally Aspirated Water Pump

    later naturally aspirated water pump when condition or service history warrants replacement Fitment/availability: 2006-2009 Legacy naturally aspirated 2.5-liter branch; confirm VIN and production split Fitment/availability: direct Add to…

    SubaruPartsDeal
  • Genuine Subaru 21111AA240 Turbo Water Pump

    turbo water pump when failed, leaking, rough, due or selected for a full timing service Fitment/availability: 2005-2009 Legacy GT 2.5-liter turbo branch within this card; confirm VIN and required gasket/seal Fitment/availability: ships i…

    SubaruPartsDeal
  • Genuine Subaru 13033AA042 Hydraulic Timing-Belt Tensioner

    hydraulic tensioner renewed with a complete due or unknown-history timing service Fitment/availability: 2000-2009 Legacy 2.5-liter timing-belt branches in this card; confirm VIN, engine and production configuration Fitment/availability: …

    SubaruPartsDeal
  • Genuine Subaru 13073AA142 Timing-Belt Idler

    smooth timing idler in the exact engine-specific position and quantity Fitment/availability: 2000-2009 Legacy 2.5-liter timing applications; position and quantity vary, so confirm VIN and engine diagram Fitment/availability: ships in 1-3…

    SubaruPartsDeal
  • Genuine Subaru 13085AA080 Timing-Belt No. 2 Idler

    2 timing idler for the exact engine-specific timing layout Fitment/availability: 2000-2009 Legacy 2.5-liter timing applications; confirm VIN, engine, position and quantity Fitment/availability: ships in 1-3 business days with Add to Cart…

    SubaruPartsDeal

As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$1,600 - $3,000
Sources for this issue (5)
  • All Wheel Drive Auto: Subaru Head Gasket Problems Explainedmedia
  • Jalopnik: These Subaru Model Years Were The Worst Affected By Head Gasket Problemsmedia
  • Fel-Pro: Solving Subaru 2.5L Head Gasket Problemsmanufacturer
  • HiRide: Subaru Outback Head Gasket Leak Symptoms Causes & DIY Diagnosismedia
  • Subimods: How to Spot a Blown Head Gasket in Your Subarumedia

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Based on 3,800 owner reports

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Community Reported
High Confidence3,800 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026
All Subaru Legacy known issues →
Community reported
890 owners
Link to this issueError Codes:P0171P0174P0300P0301P0302P0303P0304P0128

The Subaru Baja's 2.5L EJ25 naturally-aspirated engine suffers from the same chronic head gasket failure that affects all EJ25-equipped Subarus. The composite gaskets degrade over time, typically between 80,000-150,000 miles, causing external oil and coolant leaks. The Baja's engine placement and bed-over-engine design can make leaks harder to spot until they become severe. The issue affects all 2003-2006 NA Bajas. Turbo models use a different gasket design and are less susceptible.

Common Symptoms

  • Oil seepage at the base of cylinder heads
  • Coolant level slowly dropping
  • Overheating at highway speeds
  • Sweet coolant smell from engine compartment
  • Milky residue on oil filler cap
  • Bubbles in coolant reservoir

How to Fix

Replace both head gaskets with MLS (multi-layer steel) gaskets (Six Star or Subaru OEM MLS 11044AA633). Machine both cylinder heads. Replace timing belt, water pump (21111AA370), thermostat, cam seals, and valve cover gaskets while the engine is apart. Use Subaru Cooling System Conditioner. Due to Baja rarity, keep the vehicle maintained - these are appreciating in value.

What you need to fix it

Only diagnosis- and fitment-reviewed repair parts are linked here.

  • Genuine Subaru 11044AA633 cylinder head gasket

    2003-2006 naturally aspirated Subaru Baja 2.5L only after VIN/engine confirmation; quantity two required. Do not expose this path for an EJ255 turbo repair merely because a broad retailer fitment table includes multiple Baja trims.

    SubaruPartsDeal

As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$1,500 - $2,800
Sources for this issue (3)
  • 2003 Baja Replace EJ25 Head Gasket - Subaru Forester Ownersforum
  • Subaru Head Gasket Problems - Mechanic Basemedia
  • Subaru Head Gasket Problems Explained 1990-Present - Ultimate Subaruforum

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Based on 890 owner reports

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Community Reported
High Confidence890 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026
All Subaru Baja known issues →
Community reported
780 owners
Link to this issueError Codes:P0300P0301P0302P0303P0304P0305P0306

The EJ257 engine in the STI is known for rod bearing failures, especially when modified or driven hard. The stock rod bearings have thin oil clearances and can spin under high load conditions. Oil starvation during hard cornering can accelerate bearing wear.

Common Symptoms

  • Knocking noise from bottom of engine
  • Low oil pressure warning
  • Metal shavings in oil
  • Sudden loss of power
  • Engine seizure in severe cases

How to Fix

Change oil every 3,000 miles with high-quality synthetic (Motul, Rotella T6). Install oil baffle or oil pan with baffle for track use. Consider upgraded rod bearings (ACL Race) if rebuilding. Monitor oil pressure and use oil analysis to catch wear early.

Owner tips & cautions

  • WarningThis is a high-severity issue - ignoring it can lead to costly repairs or safety concerns. Address it promptly.
  • TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
Typical repair cost:$5,000 - $12,000

Research This Issue

Based on 780 owner reports

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Community Reported
High Confidence780 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026
All Subaru WRX STI known issues →
Community reported
520 owners
Link to this issueError Codes:P0300P0301P0302P0303P0304P0305P0306

The FA20 engine in 2013-2016 BRZs was subject to a safety recall (WQG-66 / NHTSA 18V-845) for defective valve springs that can fracture due to insufficient hardness. A broken valve spring causes a cylinder misfire and can lead to engine stalling. In worst cases, the broken spring allows valve-to-piston contact, causing catastrophic engine damage. Subaru recalls affected VINs for free valve spring replacement. The recall also affects the Toyota 86/Scion FR-S which shares the same engine.

Common Symptoms

  • Engine misfire (single cylinder)
  • Check engine light with misfire code (P0301-P0304)
  • Rough idle or engine shaking
  • Loss of power
  • Engine stalling at idle or low speed
  • Ticking or unusual noise from engine top end

How to Fix

Check recall status by VIN at subaru.com/owners/recalls. Recall WQG-66 (NHTSA 18V-845) covers free replacement of all intake and exhaust valve springs (16 total) at Subaru dealer. If valve-to-piston contact occurred, engine replacement is covered under the recall. Do not delay - a broken valve spring can cause catastrophic engine damage.

Typical repair cost:$0 - $0
Sources for this issue (4)
  • frs recall regarding valve spring defect - FT86CLUBforum
  • Engines Keep Failing Subaru BRZ Valve Spring Recall - The Drivemedia
  • Subaru and Toyota to Recall Engine Issues - Consumer Reportsmedia
  • Toyota Subaru Face Class-Action Lawsuit Valve Spring Recall - Center for Auto Safetymedia

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Based on 520 owner reports

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Community Reported
High Confidence520 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026
All Subaru BRZ known issues →
Community reported
420 owners
Link to this issueError Codes:P0011P0012P0016P0300P0301P0302P0303P0304P0017P0018P0019

The 3.6L EZ36 flat-6 engine uses timing chains instead of belts, but the chains and guides are not maintenance-free as often assumed. The timing chain tensioner and plastic chain guides wear over time, typically after 120,000-180,000 miles. Worn chains stretch, causing rough idle, misfires, and timing-related codes. The EZ36 uses three timing chains (one for each bank plus one for the center) making replacement a complex and labor-intensive job. If a chain jumps timing due to severe stretch or guide failure, valve-to-piston contact can cause catastrophic engine damage.

Common Symptoms

  • Rattling or chattering noise from front of engine on cold start
  • Rough idle that improves as engine warms
  • Check engine light with timing correlation codes (P0016, P0017, P0018, P0019)
  • Reduced engine performance
  • Engine misfires at idle
  • Chain noise that increases over time

How to Fix

Replace all three timing chains, chain tensioners (3 units), chain guides (6 pieces), and chain tensioner shoes. This is a major job requiring 12-16 hours of labor. Use genuine Subaru timing chain kit or quality aftermarket (OSK brand). Replace water pump, thermostat, and front crank seal while the front of the engine is disassembled. Total repair with all components: $2,500-4,000 at an independent shop.

What you need to fix it

Only diagnosis- and fitment-reviewed repair parts are linked here.

  • Genuine Subaru 13143AA071 Timing Chain

    one of three required chains after mechanical timing diagnosis Fitment/availability: 2008-2014 Tribeca 3.6-liter EZ36; current replacement for 13143AA070, confirm VIN/production date Fitment/availability: ships in 1-3 business days with …

    SubaruPartsDeal
  • Genuine Subaru 13143AA081 Timing Chain

    second timing chain after diagnosis Fitment/availability: 2008-2014 Tribeca 3.6-liter EZ36; current replacement for 13143AA080, confirm VIN/production date Fitment/availability: ships in 1-3 business days with Add to Cart live when reviewed

    SubaruPartsDeal
  • Genuine Subaru 13143AA061 Timing Chain

    third timing chain after diagnosis Fitment/availability: 2008-2014 Tribeca 3.6-liter EZ36; current replacement for 13143AA060, confirm VIN/production date Fitment/availability: ships in 1-3 business days with Add to Cart live when reviewed

    SubaruPartsDeal
  • Genuine Subaru 13142AA050 Timing Chain Tensioner

    one of three position-specific tensioners Fitment/availability: 2008-2014 Tribeca 3.6-liter EZ36, position 13142; confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart live when reviewed

    SubaruPartsDeal
  • Genuine Subaru 13142AA040 Timing Chain Tensioner

    second position-specific tensioner Fitment/availability: 2008-2014 Tribeca 3.6-liter EZ36, position 13142B; confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart live when reviewed

    SubaruPartsDeal
  • Genuine Subaru 13142AA080 Timing Chain Tensioner

    third position-specific tensioner Fitment/availability: 2008-2014 Tribeca 3.6-liter EZ36, position 13142A; confirm VIN Fitment/availability: ships in 1-3 business days with Add to Cart live when reviewed

    SubaruPartsDeal

As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$2,500 - $4,500
Sources for this issue (4)
  • 2008 3.6L Timing - Subaru B9 Tribeca SUV Forumforum
  • 3.0 H6 Timing Chain and Guide Discussion - Subaru Outback Forumsforum
  • Subaru Tribeca Bad Timing Belt or Chain Symptoms - Drivetrain Resourcemedia
  • 2006 H-6 3.0 Timing Chain renewal - Subaru B9 Tribecaforum

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Based on 420 owner reports

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Community Reported
High Confidence420 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026
Link to this issueError Codes:P0300P0301P0302P0303P0304P0305P0306

The 2006-2007 Subaru Tribeca with the 3.0L EZ30 flat-6 engine can develop head gasket leaks, typically at higher mileages (130,000-200,000 miles). While the EZ30 is generally more resistant to head gasket issues than the smaller EJ25 boxer engines, the problem does occur. The flat-6 boxer design means the head gaskets are oriented horizontally, which allows coolant to seep past the gasket under gravity when the engine is off. Symptoms include external oil leaks between the head and block, coolant consumption, and in severe cases, coolant entering the combustion chamber causing white exhaust smoke and overheating. The repair is complex due to the flat-6 layout and cramped engine bay of the Tribeca.

Common Symptoms

  • Oil seeping between cylinder head and engine block
  • Coolant level dropping without visible external leaks
  • White exhaust smoke (coolant burning in combustion chamber)
  • Heater blowing cold air intermittently (air trapped in cooling system)
  • Engine overheating
  • Sweet coolant smell from engine bay
  • Temperature gauge fluctuations

How to Fix

Typical repair cost:$2,500 - $4,000
Sources for this issue (1)
  • YouTube video by EricTheCarGuy on diagnosing and repairing Subaru EZ30 head gasket leaksvideo

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Link to this issueError Codes:P0301P0302P0303P0304

The turbocharged EJ255 has thin, cast-piston ringlands that crack under detonation. The large bore, high cylinder temperatures and a flat oil pan that lets oil slosh away from the pickup under cornering make the engine vulnerable: a single knock event (low-octane fuel, poor tune, hard driving, or oil starvation) can fracture the second ringland. Once cracked, combustion gases blow into the crankcase and oil consumption spikes to roughly a quart every 300-500 miles, with misfires and eventual catastrophic engine failure.

Common Symptoms

  • Heavy oil consumption (quart per 300-500 miles)
  • Cylinder misfire and rough running
  • Blue exhaust smoke
  • Loss of power / failed leakdown on one cylinder
  • Engine knock under load

How to Fix

Run only the required premium fuel, change oil on short synthetic intervals, address any knock/detonation immediately, and avoid extended high-boost driving on a stock bottom end. A cracked ringland is confirmed by a leakdown/compression test on the affected cylinder; the repair is a rebuild or replacement of the short block. Many owners (especially on modified cars) choose forged pistons at rebuild time as an owner-community upgrade.

Typical repair cost:$4,000 - $8,000
Sources for this issue (3)
  • 05 XT Ring land failure — Subaru Outback/Forester forumforum
  • How to Avoid Ringland Failure in your Subaru — Come and Drive Itarticle
  • Suggestions needed, piston rings are going bad — Subaru Forester Owners Forumforum

Research This Issue

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Community Reported
Medium ConfidenceVerifiedReviewed Jun 2026
All Subaru Forester known issues →

P0303 code reference

General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.

Frequently asked questions

What does P0303 mean on a Subaru?

P0303 stands for "Cylinder 3 Misfire Detected." This code means your engine's computer detected a misfire specifically in cylinder number 3 — that cylinder isn't burning its fuel-air mixture properly or completely. The computer senses misfires by watching for tiny fluctuations in crankshaft speed each time a cylinder is supposed to fire. A misfire wastes fuel, can cause a noticeable shake, rough idle or loss of power, and can flood raw fuel into the exhaust where it may overheat and damage the catalytic converter. Because the fault is isolated to one cylinder, the cause is usually something specific to that cylinder's ignition, fuel delivery, or compression. On Subaru it is documented across 8 models: Outback, Impreza, Legacy, Baja, WRX STI, BRZ, Tribeca, Forester.

What are the most common causes of P0303?

The most common causes of P0303 are: Worn or fouled spark plug in cylinder 3, Failed or weak ignition coil on cylinder 3, Bad spark plug wire or boot (where applicable), Clogged or leaking fuel injector for cylinder 3, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Wiring or connector fault at the coil or injector. The specific cause varies by vehicle.

How much does it cost to fix P0303 on a Subaru?

Published repair estimates on Subaru range from $60 to $12,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.

Related codes

  • P0300 · Random/Multiple Cylinder Misfire Detected
  • P0301 · Cylinder 1 Misfire Detected
  • P0302 · Cylinder 2 Misfire Detected
  • P0304 · Cylinder 4 Misfire Detected
  • 00522 · Engine Coolant Temperature Sensor (G62): Open Circuit / Short to Positive (B+)
  • 16500 · Engine Coolant Temp. Sensor (G62): Implausible Signal
  • 16502 · Engine Coolant Temperature Sensor (G62): Signal too High
  • 16566 · Mass Air Flow Sensor Signal Too Low (VAG)

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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.

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