P0304
P0304 Check Engine Code
Cylinder 4 Misfire Detected
On a lot of four-cylinder engines, cylinder 4 is the rearmost plug — hardest to reach, easiest to leave with a loose boot after DIY work. When that hole misfires, you get P0304: cylinder 4 misfire detected. Fuel is not burning cleanly there, so idle can feel rough and a flashing light under load still risks catalytic converter damage. Diagnosis starts with the plug, coil, and boot on cylinder 4 before chasing injectors or compression.
Symptoms
- P0304 on four-cylinder engines — very common isolated misfire code
- Rough idle, especially when cold
- May set with P0300 if counts spread to other cylinders
- After spark plug service where cylinder 4 is against the firewall
Can I keep driving with a P0304 code?
Gentle driving to the shop if the light is steady.
Flashing light — reduce throttle; misfire is active.
Common causes
- Spark plug or coil on cylinder 4
- Loose coil boot after recent plug change
- Injector on cylinder 4
- Compression loss
Often sounds like
- P0300
- P0303
If you wait
- Catalyst damage on short commutes
- Repeat misfire from a loose boot
How we check it
- 1 Cylinder 4 misfire counts
- 2 Inspect plug and boot access
- 3 Coil test
What happens next when you bring it in?
- 1 Mention recent plug work when you call — cylinder 4 is often the one that gets disturbed.
- 2 We verify misfire counts on cylinder 4 before quoting parts.
Questions we get a lot
- What does P0304 mean?
- P0304 is Cylinder 4 Misfire Detected — isolated misfire on the cylinder in the number 4 firing position for your engine, not necessarily the fourth cylinder from the front.
- Is P0304 common on 4-cylinder cars?
- Yes — on many inline fours, P0304 shows up when one coil or plug fails on the rearmost cylinder. We still confirm on scan data before replacing anything.
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