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P0128MinorCooling

P0128: How to Diagnose & Fix — Coolant Thermostat Below Regulating Temperature

164 vehicles · 29 makes · $20-$10,500 repair

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How to diagnose and fix P0128: most-reported vehicles

Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.

Subaru Outback · 1999–2010

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

Research This Issue

Based on 4,521 owner reports

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Community Reported
High ConfidenceVerified4,521 owner reportsLast reported by owners Jan 2024Reviewed Feb 2026

Toyota Tundra · 2007–2012

Community reported
4,500 owners
Link to this issueError Codes:P0128P2181

The 5.7L V8 water pump is prone to premature failure, often occurring between 40,000-70,000 miles. The water pump uses a spring-loaded seal that can fail, and internal bearing failure is common. If not addressed, this can lead to engine overheating and severe damage.

Common Symptoms

  • Coolant leak under vehicle
  • Growling noise at idle
  • Engine overheating
  • Low coolant warning light
  • Sweet smell of coolant

How to Fix

Replace the water pump along with thermostat, coolant, and serpentine belt as preventive maintenance. OEM water pump replacement costs $400-$800 including labor.

What you need to fix it

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

  • Toyota 16100-09491 genuine 5.7L engine water-pump assembly

    replace a leaking, rough, loose or otherwise test-confirmed failed 5.7L water pump Fitment/availability: 2007-2021 Toyota Tundra 5.7L 3UR-FE/3UR-FBE catalog branch; supersedes 16100-39495, 16100-39496 and 16100-09490, with VIN and engine…

    ToyotaPartsDeal

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 $400-$800 but DIY costs are typically 50-70% less
  • TipReplace the water pump along with thermostat, coolant, and serpentine belt as preventive maintenance.
  • TipConsider OEM parts for critical components like sensors and electrical parts - aftermarket can be unreliable
  • TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
Typical repair cost:$400 - $800

Research This Issue

Based on 4,500 owner reports

GoogleRedditYouTubeNHTSA
Community Reported
Medium Confidence4,500 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026

Subaru Impreza · 1999–2011

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

Research This Issue

Based on 4,200 owner reports

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Community Reported
High Confidence4,200 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026

Subaru Legacy · 2000–2009

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

Research This Issue

Based on 3,800 owner reports

GoogleRedditYouTubeNHTSA
Community Reported
High Confidence3,800 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026

Toyota Prius · 2004–2009

Community reported
2,100 owners
Link to this issueError Codes:P0217P0128

The electric inverter coolant pump (G9020-47031) on Gen 2 Prius fails without warning, causing the inverter to overheat. If not caught quickly, overheating can destroy the inverter assembly ($3,000+ repair). Pump typically fails between 100,000-180,000 miles.

Common Symptoms

  • No warning light until overheating occurs
  • Inverter overheating warning message
  • Vehicle enters limp mode
  • Loss of hybrid drive capability
  • Red triangle warning light

How to Fix

Replace the inverter coolant pump (Toyota 04000-32528 or aftermarket G9020-47031). This is a preventive maintenance item - replace at 100,000-120,000 miles before failure. The pump is located on the passenger side of the engine bay. Also replace the inverter coolant (Toyota Super Long Life Coolant) every 100,000 miles. DIY difficulty is moderate.

What you need to fix it

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

  • Toyota G9020-47031 drive-motor inverter cooler water pump

    replace a confirmed failed inverter coolant pump after the free recall route has been checked Fitment/availability: 2004-2009 Prius 1.5L inverter cooling circuit, replacing G9020-47030; purchase only after VIN recall status and diagnosis…

    ToyotaPartsDeal

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:$200 - $600
Sources for this issue (1)
  • https://priuschat.com/threads/inverter-coolant-pump-replacement.156789/

Research This Issue

Based on 2,100 owner reports

GoogleRedditYouTubeNHTSA
Community Reported
High Confidence2,100 owner reportsLast reported by owners Feb 2026Reviewed Feb 2026

Find P0128 by make

Browse every documented model and issue for your make.

Audi · 6 issuesBMW · 5 issuesBuick · 2 issuesCadillac · 13 issuesChevrolet · 22 issuesChrysler · 11 issuesDodge · 16 issuesFord · 27 issuesGMC · 5 issuesHonda · 7 issuesHyundai · 4 issuesInfiniti · 1 issueJaguar · 1 issueJeep · 8 issuesKia · 5 issuesLincoln · 2 issuesMazda · 6 issuesMercedes-Benz · 5 issuesMINI · 6 issuesNissan · 5 issuesOpel · 1 issuePorsche · 1 issueRAM · 2 issuesSEAT · 2 issuesSkoda · 1 issueSubaru · 7 issuesToyota · 17 issuesVolkswagen · 9 issuesVolvo · 3 issues

P0128 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 P0128 mean?

P0128 stands for "Coolant Thermostat Below Regulating Temperature." This code means the engine isn't reaching its proper operating temperature within the expected time, so the coolant stays below the thermostat's regulating temperature. The computer estimates how warm the coolant should be (based on run time, airflow, and intake air temperature) and compares it to the actual reading. When the engine warms too slowly or never gets hot enough, P0128 sets. The most common cause is a thermostat stuck open, which can cause reduced heater output, poorer fuel economy, and higher emissions.

What are the most common causes of P0128?

The most common causes of P0128 are: Thermostat stuck open or opening too early, Faulty engine coolant temperature sensor, Low coolant level, Cooling fan running constantly (overcooling), Wrong-temperature-rating thermostat installed, Corroded or loose coolant temperature sensor connector, Air pockets in the cooling system, Faulty PCM/ECM (rare). The specific cause varies by vehicle.

How much does it cost to fix P0128?

Repair costs for P0128 range from $20 to $10,500, depending on the vehicle and root cause.

Which vehicles are affected by P0128?

Au7o has documented P0128 across 164 vehicle models from 29 manufacturers: Audi, BMW, Buick, Cadillac, Chevrolet, Chrysler, Dodge, Ford, GMC, Honda, Hyundai, Infiniti, Jaguar, Jeep, Kia, Lincoln, MINI, Mazda, Mercedes-Benz, Nissan, Opel, Porsche, RAM, SEAT, Skoda, Subaru, Toyota, Volkswagen, Volvo.

Related codes

  • P0120 · Throttle/Pedal Position Sensor Circuit Malfunction
  • P0121 · Throttle/Pedal Position Sensor Circuit Range/Performance
  • P0122 · Throttle/Pedal Position Sensor/Switch 'A' Circuit Low Input
  • P0123 · Throttle/Pedal Position Sensor/Switch 'A' Circuit High Input
  • P00B7 · Engine Coolant Flow Low/Performance
  • P0117 · Engine Coolant Temperature Sensor Circuit Low
  • P0118 · Engine Coolant Temperature Sensor Circuit High
  • P0217 · Engine Overtemperature Condition
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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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