Ignition

P0304 — Cylinder 4 Misfire Detected

Severity: medium Drive with care, to a shop

What P0304 means

The engine control unit detects misfires by watching for uneven crankshaft speed through the crankshaft position sensor. P0304 means cylinder 4 is not burning its air-fuel mixture properly. The cause can be missing spark, wrong fuel delivery, an air leak, or lost compression.

Symptoms

  • Check engine light on or flashing
  • Rough idle and engine vibration
  • Hesitation or stumbling during acceleration
  • Loss of power under load
  • Poor fuel economy
  • Smell of unburned fuel from the exhaust

Possible causes

  1. Worn or faulty spark plug in cylinder 4
  2. Failed ignition coil on cylinder 4
  3. Clogged or leaking fuel injector on cylinder 4
  4. Vacuum leak near the cylinder 4 intake runner (gasket, cracked hose)
  5. Low compression in cylinder 4 (valve, rings, head gasket)
  6. Loose or damaged coil boot or poor connector contact

How to diagnose

  1. Read all codes and freeze frame data, and check per-cylinder misfire counters
  2. Inspect the cylinder 4 spark plug, coil, connector and wiring
  3. Swap the coil (and plug) with a neighboring cylinder, clear the code and see whether the misfire follows
  4. Test the injector: listen for clicking, measure resistance, check cylinder balance
  5. Check the intake for vacuum leaks, ideally with a smoke machine
  6. Run a compression test if the ignition and fuel checks pass
  7. Do a cylinder leak-down test if compression is low

Common fixes

  • Replace the cylinder 4 spark plug (ideally the whole set)
  • Replace the cylinder 4 ignition coil
  • Clean or replace the fuel injector
  • Fix the vacuum leak by replacing the gasket or hose
  • Repair the engine mechanically if compression is low

Questions and answers

What does P0304 mean?

The ECU has detected a misfire in cylinder 4. The most common causes are a bad spark plug or ignition coil, followed by the injector, vacuum leaks, or internal engine wear.

Can I drive with P0304 and a flashing check engine light?

It's best not to. A flashing light signals a severe misfire that can quickly damage the catalytic converter. Have the car towed or drive only a very short distance.

How do I tell if it's the coil or the plug?

Swap the coil with one from another cylinder and clear the code. If the code changes to another cylinder, the coil is bad; if it stays on cylinder 4, check the plug, injector and compression.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.