P0304 — Cylinder 4 Misfire Detected
The engine control module has detected a combustion misfire isolated to cylinder 4.
Read this first
What the P0304 code means
P0304 flags misfires at cylinder 4. On four-cylinder engines this is the rearmost cylinder on many layouts — verify the engine's numbering before testing.
The process mirrors every single-cylinder misfire: confirm which physical cylinder is #4 for the vehicle, then test its spark, fuel, and compression.
Common symptoms
- Check-engine light on or flashing
- Rough idle and vibration
- Hesitation or stumble when accelerating
- Slight power loss
- Occasional fuel odor from unburned fuel in severe cases
Possible causes
- Worn, fouled, or gapped-wrong spark plug at cylinder 4
- Failing ignition coil
- Clogged or weak fuel injector
- Low compression — valves, rings, or head gasket
- Vacuum leak near that cylinder's runner
- Wiring or connector fault at that coil/injector
Diagnostic considerations
Swap coils to confirm before replacing. Compare the plug to others — a single oil-fouled plug can also reveal a sealing issue specific to that cylinder.
If the misfire started after recent engine work (plug replacement, intake service), double-check the connector seating and plug installation at that cylinder first.
What a technician may inspect
- Spark plug at cylinder 4: condition, gap, seating
- Coil swap test with another cylinder
- Injector balance and delivery
- Compression and leak-down for cylinder 4
- Any recent-work areas near that cylinder
Common mistakes
- Assuming cylinder 4's physical position without verifying the numbering
- Replacing coils and plugs together without isolating which failed
- Ignoring an oil-fouled plug, which points to a different root cause
When professional inspection is appropriate
Related codes
Related glossary terms
FAQ
Frequently Asked Questions
Where is cylinder 4 on my engine?
It varies by manufacturer and engine orientation. Check service data for your specific engine before testing — cylinder numbering is not standardized across brands.
Is P0304 serious?
The code itself isn't, but the cause can be. A steady light allows a careful drive to a shop; a flashing light means the active misfire risks catalytic converter damage.
What's the most common fix?
Statistically, ignition — a worn plug or weak coil at that cylinder. Confirm with a coil swap and plug inspection before replacing anything.
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