P0304 Code Meaning and Next Steps

First checks take 10 to 20 minutes for the first checks. No special tools are usually needed for the first checks.

Can you keep driving?

Stop driving if any of these apply

!The check-engine light is flashing.
!The engine is shaking badly, stalling, or struggling to accelerate.

If the light is steady and the vehicle still drives normally: Maybe, but only for a very short distance if the engine still runs smoothly.

What to check first

Step-by-step checks

1
Safety first
If the check-engine light is flashing or the engine is shaking badly, stop driving before you inspect anything else
2
Free - no tools
Inspect the spark plug, coil, connector, and injector area for cylinder 4 first
3
Basic tool needed
Look for loose connectors, vacuum leaks, or anything recently left disconnected in the engine bay
4
Basic tool needed
If scan data is available, compare misfire counts and fuel trims before replacing parts
5
Basic tool needed
If the fault follows a swap test, the confirmed part is a much stronger suspect

If the code returns

-If the misfire stays on cylinder 4 after a swap test, compression or injector testing becomes more important.
-If a lean or fuel-trim code appears too, look harder for an air leak or fuel-delivery issue.
-If the code returns immediately after a clear, focus on the active fault instead of the stored history.

Background

What this code means

P0304 is a generic OBD-II cylinder-specific misfire code.

The code points you toward cylinder 4, but the root cause can still be ignition, fuel, air, or a mechanical issue.

Diagnosis

Common causes

Most common

Ignition weakness

Worn plugs, weak coils, or poor coil connections are among the most common first checks.

Common

Fuel delivery issue

A clogged injector or local fuel-delivery fault can leave one cylinder misfiring.

Common

Unmetered air

A vacuum leak near the intake can create a lean spot that shows up as a misfire.

Possible

Mechanical fault

Low compression, valve sealing issues, or timing problems can cause a persistent cylinder misfire.

Avoid these mistakes

What not to do

xDo not keep driving if the check-engine light is flashing or the engine is shaking badly.
xDo not replace several ignition parts at once without a basic inspection or swap test.

Parts

Parts that may need replacing

PartTypical costNotes

Spark plug$10-$40Often the cheapest confirmed fix if the plug is worn or fouled.

Ignition coil$30-$120Relevant when the fault follows the coil or the coil test fails.

Injector or intake seal repair$20-$180Only after air-leak or injector evidence supports it.

Related OBD codes

P0300

P0300 usually means the engine is detecting random or multiple-cylinder misfires.

P0303

P0303 means cylinder 3 is misfiring.

P0305

P0305 usually means cylinder 5 is misfiring.

P0351

P0351 usually means the ignition coil A circuit is not behaving as expected.

Source notes

Generic OBD-II (SAE J1979 / ISO 15031-5). P0304 was expanded around common cylinder-specific misfire patterns, with conservative guidance focused on ignition, fuel, air, and compression checks.

This guide is written as a generic multi-make reference, so bulletin history, sensor locations, and repair order can still change by manufacturer and engine family.

This is generic OBD-II guidance and should not override vehicle-specific service information. Exact diagnosis and repair steps vary by make, engine family, and model year.

Last reviewed: 2026-04-10

Reference: Open reference

P0304 - P0304 Usually Means Cylinder 4 Is Misfiring