P0308: Cylinder 8 Misfire Detected

What does P0308 mean?

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. P0308 means more than 90 % of the detected misfires fall on cylinder 8. 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. On V8 engines GM bulletins name loose coil connectors and a loose coil ground on the cylinder head.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

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

Common causes

• High Ignition coil of cylinder 8 faulty (coil, connector, ground)
• High Spark plug of cylinder 8 worn, sooted, cracked or wrongly gapped
• Medium Injector of cylinder 8 clogged or leaking, or its control
• Low Low compression on cylinder 8 (valve, rings, head gasket) or local false air
• Low Water or oil in the plug well

Likelihood is a general estimate across vehicles; your make and model may differ.