Misfire
An engine cylinder that fails to combust properly — from a weak spark, bad fuel delivery, or low compression — causing a rough idle, hesitation, or a flashing check engine light.
The engine's computer counts crankshaft speed fluctuations to detect a misfiring cylinder. A steady misfire sets a code like P0301 (cylinder 1); a flashing MIL during a misfire means it's severe enough to damage the catalytic converter and driving should stop.
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