P0304 on Honda: How to Diagnose & Fix
Cylinder 4 Misfire Detected
10 Honda models · 19 documented issues · 9 critical · $20-$8,000 repair
P0304 on Honda by model: symptoms, diagnosis and fix
Variable Cylinder Management (VCM) deactivates 3 rear cylinders during light load, but insufficient pressure allows oil to bypass piston rings into combustion chamber. Causes excessive oil consumption (1 quart per 1,000 miles), fouled spark plugs, catalytic converter damage, and premature piston ring wear. Honda settled class-action lawsuit covering 1.6-1.87 million vehicles.
Common Symptoms
- Excessive oil consumption (1 qt per 1,000 miles or more)
- Engine misfire codes (P0300-P0304, typically cylinders 1-4)
- Fouled/oil-soaked spark plugs
- Rough idle and vibration
- Check engine light (misfire and catalyst codes)
- White smoke from exhaust on startup
- Failed catalytic converters (P0420/P0430)
How to Fix
Install VCM disable device (VCMuzzler, VCMTuner) which prevents cylinder deactivation and stops oil consumption. Replace fouled spark plugs after installing disabler. Honda settled class-action providing 8-year warranty extension for engine misfire repairs. Severe cases may require piston ring replacement ($3,000-$5,000). 2014+ models received redesigned engine without VCM.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Honda/NGK 9807B-5517W IZFR5K11 spark plug, eachOEMHonda 9807B-5517W / NGK IZFR5K11
Early-generation plug branch only after inspection identifies a fouled plug and exact catalog application Fitment/availability: 2005-2007 U.S. Honda Odyssey 3.5L applications in this issue; Honda catalogs 9807B-5517W across 2005-2010 but…
- Genuine Honda/NGK 12290-R70-A01 ILZKR7B11 spark plug, eachOEMHonda 12290-R70-A01 / NGK ILZKR7B11
VCM plug-fouling branch after code and plug inspection identifies replacement Fitment/availability: 2008-2010 U.S. Honda Odyssey EX-L/Touring with VCM; select 12290-R70-A01.
- Genuine Honda/NGK 12290-R71-L01 DILZKR7A11G spark plug, eachOEMHonda 12290-R71-L01 / NGK DILZKR7A11G
Fourth-generation plug-fouling branch after P0301-P0304 and physical plug diagnosis Fitment/availability: 2011-2017 U.S. Honda Odyssey all trims; sold individually and six are required for a full set.
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
- TipCheck oil level every fuel fill-up on VCM-equipped models - carry a quart of oil in the van
- WarningDon't ignore misfire codes - prolonged misfires destroy catalytic converters ($2,000+ repair)
Sources for this issue (2)
The 1.5L turbocharged Earth Dreams direct-injection engine (L15B7) accumulates heavy carbon deposits on intake valves and ports because fuel is injected directly into the cylinder, bypassing the intake valves. Unlike port-injection engines, no fuel washes the intake valves clean. Owners report symptoms beginning at 40,000-60,000 miles. Carbon buildup restricts airflow, causing rough idle, misfires, reduced power, and reduced fuel economy. This is inherent to direct-injection technology and affects all 10th gen Civic 1.5T variants (LX through Si). Honda addressed this in 11th gen with port+direct injection (GDI).
Common Symptoms
- Rough idle or stumbling at idle
- Misfires on cold start (P0300-P0304)
- Hesitation or stumbling during acceleration
- Reduced power output compared to new
- Decreased fuel economy (2-4 mpg reduction)
- Hard cold starts
- Occasional rough running that improves when warm
How to Fix
Walnut blasting (walnut shell media blast cleaning) of intake valves every 40,000-60,000 miles is the industry-standard solution: $300-500 at independent shops, $500-800 at dealers. Chemical intake cleaners (CRC GDI IVD Intake Valve Cleaner #05319, Liqui-Moly Valve Clean) used every 10,000 miles can slow buildup but do not fully remove existing deposits. Top-tier gasoline (Shell V-Power, Chevron Techron) contains detergent additives that help prevent buildup. Catch-can installation ($80-150) reduces oil vapor contributing to buildup.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- CRC GDI IVD Intake Valve & Turbo Cleaner 05319, 11 ozOEMCRC 05319
Manufacturer-directed chemical cleaning option for diagnosed or suspected light-to-moderate GDI intake deposits Fitment/availability: 2016-2021 Civic 1.5T only after ruling out bulletin/software, injector, ignition, boost and mechanical …
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
- TipCivicX.com consensus: Schedule walnut blast cleaning at 50,000 miles regardless of symptoms - visible buildup starts at 40k miles and only gets worse. Independent shops charge $300-400 vs dealer $600+
Sources for this issue (3)
Two independently filed NHTSA complaints on near-new 2025 Civics describe the same failure: sudden rough idle and near-stall shortly after startup at roughly 2,000 and 9,000 miles. In the first, the dealer found active misfires on all four cylinders with all four spark plugs damaged on removal and diagnosed four faulty fuel injectors, replacing the full set. In the second, the dealer found active misfires on two cylinders, with dark grey smoke and a sulfur smell from the exhaust and shaking severe enough to crack the engine cover. This is separate from the 24V763000 high-pressure fuel pump recall — the failing component here is the direct injectors themselves, and the fuel-trim disturbance shows up as fouled plugs rather than a leak.
Common Symptoms
- Sudden rough idle and shaking at startup on a near-new car
- Active misfires on multiple cylinders
- Fuel-fouled or damaged spark plugs on removal
- Dark grey exhaust smoke and sulfur smell
- Shaking severe enough to crack the engine cover
How to Fix
Scan for misfire codes (P0300 plus the cylinder-specific P0301-P0304) and have the dealer perform an injector balance/leak-down test before parts are thrown at it — a wet, fuel-fouled plug on a cylinder with a leaking injector is the diagnostic tell. Replacement injectors and plugs are covered under Honda's 3-year/36,000-mile new-vehicle warranty and the 8-year/80,000-mile federal emissions warranty; do not accept a plugs-only repair, because new plugs on a leaking injector will foul again within weeks. Have the dealer confirm the high-pressure fuel pump recall 24V763000 status on the same visit, since fuel-pressure faults can present similarly.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
Honda bulletin 19-038 documents specified misfire/rich-mixture codes (P0300-P0304, P0172) or engine whirling noise on 2016–2018 Civics with the 1.5L engine. Applicability and coverage require a VIN check.
Common Symptoms
- Misfire or rich-mixture fault codes
- Whirling noise from the engine
How to Fix
Ask Honda to check VIN eligibility and prior repair history. Complete 18-137 first only if it applies to this VIN and has not already been completed. Bulletin 19-038 then separates a fouled-spark-plug branch from camshaft/rocker diagnosis. Follow the applicable technician procedure; if the specified conditions are not found or symptoms remain, continue normal troubleshooting. Do not assume free coverage today.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- NGK 95112 / ILZKAR8H8S spark plug — non-Si repair branch onlyFits 2016-2018 · EX-T / EX-L / Touring / SportOEM12290-59B-003Cross-refNGK 95112, ILZKAR8H8S
Confirm VIN and current service parts before purchase.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (1)
2016-2018 Pilots suffer from debris in direct-injection fuel system causing internal wear or clogging inside injectors. Causes misfire codes (P0300-P0306), catalyst efficiency codes (P219A/B, P0420/P0430), rough idle, and stalling. TSB 20-100 addresses this defect. Honda extended warranty to 10 years/150,000 miles for 2016-2017 models.
Common Symptoms
- Check engine light with misfire codes (P0300-P0306)
- Catalyst efficiency codes (P0420, P0430, P219A, P219B)
- Rough idle or engine stumbling
- Engine stalling at stops
- Poor fuel economy
- Fuel smell at idle
- Loss of power under acceleration
How to Fix
Visit Honda dealer for diagnosis using TSB 20-100. For vehicles with these codes, Honda will replace fuel injectors and catalytic converters under extended warranty (10 years/150,000 miles for 2016-2017). PCM software update also available. Keep documentation of all misfires and check engine lights to support warranty claim.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Honda 16010-RLV-315 six-piece fuel-injector setOEMHonda 16010-RLV-315
Retail fallback only after Honda warranty/VIN review and diagnosis confirm the injector branch Fitment/availability: 2016-2018 U.S.
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
- TipHonda extended warranty to 10 years/150,000 miles for 2016-2017 models with these codes - document all issues
- TipIf dealer denies warranty claim, reference TSB 20-100 specifically and escalate to Honda corporate
- WarningDo not ignore misfire codes - continued driving can damage catalytic converters ($1,500-$3,500 to replace)
- WarningThese codes often misdiagnosed as VCM issues - verify with dealer using TSB 20-100 for correct diagnosis
Sources for this issue (2)
On the J35 V6, the rubber valve cover gaskets and the integrated spark plug tube seals harden and shrink with age and heat, allowing oil to weep onto the hot exhaust manifolds (burning-oil smell) and, more seriously, to pool down inside the spark plug wells. Oil saturates the ignition coil boots and shorts the spark, producing misfires and a Check Engine Light. The rear (firewall-side) bank is hardest to access, so the leak is often neglected until misfires appear.
Common Symptoms
- Burning oil smell from the engine bay
- Oily residue/grime on top of the valve covers
- Oil pooled in the spark plug wells
- Rough idle, misfire, and Check Engine Light
- Occasional blue smoke / slightly higher oil consumption
How to Fix
Replace both valve cover gaskets and the spark plug tube seals (gasket sets include the tube seals); clean oil from the spark plug wells and inspect/replace any oil-soaked ignition coil boots and fouled spark plugs. Parts are inexpensive ($40-$80 per set) but labor is significant on the rear bank.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Fel-Pro VS50773R complete valve-cover gasket setOEMFel-Pro VS50773R / PartsGeek 29982-05418108
Confirm package contents on arrival.
- Genuine Honda 12030-5G0-000 front head-cover gasket packageOEMHonda 12030-5G0-000
2016-2020 front/radiator-bank gasket and tube-seal branch Fitment/availability: 2016-2020 U.S. Honda Pilot 3.5L V6 front head cover; package includes the front cover gasket, tube seals and sealing washers.
- Genuine Honda 12050-5G0-000 rear head-cover gasket packageOEMHonda 12050-5G0-000
2016-2020 rear/firewall-bank gasket and tube-seal branch Fitment/availability: 2016-2020 U.S. Honda Pilot 3.5L V6 rear head cover; package includes the rear cover gasket, tube seals and screw grommets.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (2)
P0304 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 P0304 mean on a Honda?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On Honda it is documented across 10 models: Odyssey, Civic, Pilot, Accord, Fit, Ridgeline, Insight, CR-V, Passport, Crosstour.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a Honda?
Published repair estimates on Honda range from $20 to $8,000 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.
Looking for P0304 on a different make?
View P0304 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.