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Spark delivered at the wrong moment relative to the piston. On a modern engine timing is calculated rather than set, so this is almost never somebody having mistimed anything — it is the module acting on a reference that has moved.

A stretched timing chain or a slipped belt is the mechanical version, shifting the camshaft's relationship to the crankshaft so every calculation built on it is offset. The signature is a crankshaft-to-camshaft correlation code stored alongside the misfire. Too early gives detonation and knock retard; too late gives poor economy, high exhaust temperature and a flat engine. Check the mechanical timing marks before adjusting anything electronic, since no module compensates for a chain that has jumped a tooth.