P0304 Check Engine Code

What's going on

P0304 is misfire on cylinder 4 — on many four-cylinder engines, cylinder 4 is the rearmost plug and often the hardest to service; previous DIY tune-ups sometimes leave a loose boot or cracked plug there. Same ignition-first diagnosis as other cylinder-specific codes.

Symptoms

• P0304 on four-cylinder engines — very common isolated misfire code
• Rough idle, especially when cold
• May set with P0300 if counts spread
• After spark plug service where cylinder 4 is against the firewall

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

How we check it

1. 1 Cylinder 4 counts
2. 2 Inspect plug and boot access
3. 3 Coil test

What happens next when you bring it in?

1. 1 Mention recent plug work when you call — cylinder 4 is often the one that gets disturbed.
2. 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 position for your engine’s firing order.

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.

Related problems

• P0300 Check Engine Code
• Rough Idle
• Check Engine Light