P0304 Check Engine Code
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.
Often confused with
- P0300
- P0303
Common causes
- Spark plug or coil on cylinder 4
- Loose coil boot after recent plug change
- Injector on cylinder 4
- Compression loss
If you ignore it
- Catalyst damage on short commutes
- Repeat misfire from a loose boot
Repair overview
- Cylinder 4 misfire counts
- Inspect plug and boot access
- Coil test
What happens next
- Mention recent plug work when you call - cylinder 4 is often the one that gets disturbed.
- We verify misfire counts on cylinder 4 before quoting parts.
Questions
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.