Powertrain, generic · Ignition system or misfire

P0312 Cylinder 12 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. P0312 means more than 90 % of the detected misfires fall on cylinder 12. 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. Cylinder 12 exists only on twelve-cylinder engines; no manufacturer document for this code was found, the monitor works as described.

Likely causes

Ignition coil of cylinder 12 faulty (coil, connector, ground)
Spark plug of cylinder 12 worn, sooted, cracked or wrongly gapped
Injector of cylinder 12 clogged or leaking, or its control
Low compression on cylinder 12 (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.

Parts involved

ignition coil cylinder 12spark plug cylinder 12fuel injector cylinder 12

Related codes

P0300P0362P0212P0316P0420

References: law.cornell.edu, gsi.ext.gm.com, newtonnet.co.uk.