P0304: Cylinder 4 Misfire Detected
Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.
What P0304 means
The ECM counted repeated misfires on cylinder 4. This page focuses on mechanical causes: when parts swaps don't fix it.
What sets it
Set by the crankshaft-speed misfire counter for cylinder 4 passing its calibrated threshold. A misfire that survives coil, plug and injector swaps usually means the cylinder isn't sealing or breathing properly.
Symptoms
- Consistent miss that is present at all engine speeds
- Possible ticking or puffing noise on a valve problem
- Rough idle that doesn't improve when warm
Is it safe to drive?
General guidance, not a guarantee: Minimize driving if the light flashes. A mechanical misfire won't fix itself, and the unburned fuel stresses the converter. If in doubt, have it towed and diagnosed.
Common causes
Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.
- Spark plug, coil or boot: always rule these out first
- Injector
- Low compression: burnt or bent valve, worn rings, head gasket: the main subject of this page
- Worn cam lobe or collapsed lifter (where applicable)
- Intake leak near cylinder 4
Test before you replace: diagnostic sequence
General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.
- Swap the coil and plug to another cylinder and confirm the misfire stays on cylinder 4.
- Confirm the injector is pulsing and contributing.
- Run a relative compression test (scan tool or scope current ramp) to compare cylinders quickly without pulling every plug.
- Run a cranking compression test on all cylinders and compare them with each other and with the manufacturer specification.
- If cylinder 4 is low, run a leak-down test and listen at the intake, exhaust and oil filler, and watch the coolant, to locate the leak path.
- Check for coolant consumption or white exhaust if a head gasket is suspected.
- Explain mechanical findings to the customer with the measurements before quoting major work.
- Verify any repair with misfire counters after a drive.
Bench note: Compare compression readings with each other as well as with the spec. One cylinder well below its neighbors matters even if every reading is "in range".
Related codes
FAQ
What does a leak-down test tell me that compression doesn't?
Where the air escapes: intake means an intake valve, the tailpipe means an exhaust valve, the crankcase means rings, and bubbles in the coolant suggest a head gasket.
Can a timing chain cause one-cylinder misfire?
Usually not. Timing problems affect all cylinders and tend to set cam/crank correlation codes such as P0016. A single-cylinder mechanical misfire is more often a valve or ring problem.
Is a mechanical misfire worth repairing?
It depends on the vehicle's value and the measured findings. Get the leak-down result first so the decision is based on facts.
Sources
- SAE J2012, Diagnostic Trouble Code Definitions (standard page; code titles only)
- California Code of Regulations, 13 CCR 1968.2, OBD II requirements (via Cornell LII)
- US EPA, "What To Do With the Check Engine Light" (EPA420-F-00-041)
Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.