P0304 on Pontiac: How to Diagnose & Fix
Cylinder 4 Misfire Detected
1 Pontiac model · 1 documented issue · 1 critical · $1,000-$2,500 repair
P0304 on Pontiac by model: symptoms, diagnosis and fix
The first-generation 2.2L Ecotec L61 in 2002-2005 Sunfire (and Cobalt, Cavalier, Saturn Ion) is prone to head gasket failure and cylinder head cracking — particularly if overheated once. Aluminum head warps easily. Often presents as bubbles in coolant reservoir, white smoke, and overheating.
Common Symptoms
- white smoke
- overheating
- bubbles in coolant
- milky oil
- rough idle after warm-up
How to Fix
Pressure-test cooling system to confirm. Head gasket replacement ~$1,000-$1,800 — but ALWAYS have the head pressure-tested and resurfaced (~$150-$250 machine shop) since cracking is so common. If head is cracked, a used head is often the most economical replacement ($200-$400 from yard). Driving while overheating destroys the head; tow it.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Fel-Pro HS 26223 PT-1 cylinder-head gasket set
2002-2005 Pontiac Sunfire 2.2L DOHC Ecotec L61; head bolts are not included and the listing requires TCS46041. Fitment/availability: Fourteen in stock and add-to-cart live at review
- Replacement 698-001 fourteen-piece L61 head-bolt set
2002-2005 Pontiac Sunfire 2.2L DOHC Ecotec L61; includes ten long and four short bolts and interchanges with ES72196/HB32201. Not ES72170.
- Fel-Pro TCS 46041 timing-cover gasket set
2002-2005 Pontiac Sunfire 2.2L L61; verify production details and contents against the removed timing-cover seals.
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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 Pontiac?
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 Pontiac it is documented across 1 model: Sunfire.
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 Pontiac?
Published repair estimates on Pontiac range from $1,000 to $2,500 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.