CodeXOBD

P0304 — Cylinder 4 Misfire Detected

P0304 means cylinder 4 is not firing correctly. The same ignition-fuel-compression diagnostic approach applies.

High

The engine runs rough with reduced power. A flashing check engine light means active misfires that can destroy the catalytic converter. This code will cause an emissions test failure.

$10–$500Parts, typical range
0.3–3 hLabour
estimateNot a quote · 2026 basis

What P0304 means

P0304 is the cylinder 4 equivalent in the P030x misfire family. On a four-cylinder engine, cylinder 4 is the last in the firing order. The module detects the misfire through crankshaft speed analysis and sets this code when the pattern is consistent. Individual cylinder misfires are among the most common codes and are usually cheap to fix if diagnosed in the right order.

Technical summary
SystemPowertrain — ignition and misfire
How a misfire is detectedThe crankshaft sensor gives position and speed, the camshaft sensor gives which stroke, and the module times the coil and injector for each cylinder from both. It detects a misfire by the small deceleration of the crankshaft that follows a cylinder which failed to fire, and attributes it to a cylinder when the pattern is consistent.Crank sensorposition, speedCam sensorwhich strokeModuletimes the sparkCoil & injectorper cylinderA misfire is measured as crankshaft deceleration, not as a missing spark

Symptoms you may notice

What causes P0304

Spark plug in cylinder 4 is fouled or worn

Very likelylow costeasy

The most common and cheapest cause.

Ignition coil for cylinder 4 has failed

Likelylow costeasy

Swap with another cylinder to confirm.

Fuel injector for cylinder 4 clogged or failed

Possiblemedium costmoderate

A clogged or dead injector starves cylinder 4 of fuel.

Low compression in cylinder 4

Rarehigh costhard

Mechanical failure — least common but most expensive.

How to diagnose it

1

Swap the ignition coil

Move the cylinder 4 coil to another cylinder and see if the misfire follows.

Tools: Basic hand toolsIf it passes: Misfire stays on cylinder 4. Continue to step 2.If it fails: Misfire followed the coil — replace it.
2

Inspect the spark plug

Remove and inspect the cylinder 4 spark plug.

Tools: Spark plug socketIf it passes: Plug looks normal. Continue to step 3.If it fails: Replace the plug.
3

Check compression

Test compression in cylinder 4 against the others.

Tools: Compression gaugeExpected: Within 10% of other cylindersIf it passes: Compression is normal. Check the injector.If it fails: Low compression — mechanical diagnosis needed.

Related codes

P0300P0301P0302P0303

Common questions

Should I replace all spark plugs if one is bad?

If the plugs are near their service interval, yes — do them all. If they were recently replaced, just do the faulty one and keep the others.

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)

This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.