Misfire

Each cylinder needs the right air, fuel and spark at the right time. If one is missing, that cylinder fails to fire properly. Common causes are worn spark plugs, failing coils, leaking or clogged injectors, vacuum leaks and low compression. OBD-II detects misfires by monitoring tiny changes in crankshaft speed. Codes P0300 (random) and P0301 to P0312 (by cylinder) identify them.

Misfires can come from ignition (plugs, coils), fuel (injectors, pressure), air (vacuum leaks) or mechanical faults (low compression). Codes P0300 to P0312 identify random or cylinder-specific misfires. Misfires are often intermittent: they may happen only when cold, under load or in wet weather. The OBD system counts misfire events per cylinder; scan tools can show these counters in live data or Mode $06, which helps locate a cylinder before a code is even stored.

Every time a cylinder fires, the crankshaft speeds up slightly. When the expected acceleration is missing, the control unit counts a misfire for that cylinder. The control unit detects misfires by watching tiny variations in crankshaft speed: a cylinder that does not fire slows the crankshaft briefly.

Severe misfires send unburnt fuel into the exhaust and can overheat the catalytic converter. A flashing engine warning light means reduce load and get the car checked promptly. A flashing check engine light means a severe misfire that can damage the catalytic converter. Reduce load and have the car checked as soon as possible.