Powertrain, generic · Ignition system or misfire

P0304 Cylinder 4 Misfire Detected

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Ford 0.9–1.5 %) it comes on after two trips. Ford and GM may shut off a misfiring cylinder's injector to protect the catalyst. P0304 means more than 90 % of the detected misfires fall on cylinder 4. Usual causes are the ignition coil or spark plug of that cylinder, its injector, or low compression; a quick test is to swap coil or plug with a neighbouring cylinder and see whether the misfire moves. Bulletins for four-cylinder engines name internal coil faults (Mazda) and plugs cracked during installation (FCA).

Likely causes

Ignition coil of cylinder 4 faulty (coil, connector, ground)
Spark plug of cylinder 4 worn, sooted, cracked or wrongly gapped
Injector of cylinder 4 clogged or leaking, or its control
Low compression on cylinder 4 (valve, rings, head gasket) or local false air
Water or oil in the plug well

Symptoms

  • Check engine light on, flashing at a catalyst-damaging rate
  • Rough idle, jerking, vibration
  • Loss of power, hesitation
  • Smell of unburnt fuel, higher consumption

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–800
Time0.3–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

What to check first

  1. Swap the coil with another cylinder and see if the misfire follows
  2. Inspect the spark plug
  3. Check injector operation
  4. Do a compression test

Parts involved

ignition coil cylinder 4spark plug cylinder 4fuel injector cylinder 4

Related codes

P0300P0354P0204P0316P0420

References: law.cornell.edu, gsi.ext.gm.com, saabwisonline.com, static.nhtsa.gov.