Misfire
Definition
A misfire is a combustion event that fails or is too weak, which the ECU detects as a momentary slowdown of the crankshaft between firing pulses using the crank sensor's tooth timing. It counts misfires per cylinder over 200 and 1000 revolution windows; exceeding the catalyst-damage threshold flashes the MIL, while the emissions threshold sets a steady lamp. Causes fall into three groups: ignition (plug, coil, wiring), fuel (injector, pressure, lean mixture from a vacuum leak) and mechanical (low compression, valve, head gasket, timing). Use the misfire counters in live data, swap coil and plug to a neighbouring cylinder, check fuel trim, and check compression if the fault stays with the cylinder. Codes: P0300 (random/multiple), P0301-P0312 (cylinder 1-12), P0313 (misfire with low fuel), P0316 (misfire on start-up), plus manufacturer counters such as VAG P130A cylinder deactivation.