Cause
Crankshaft variation relearn not performed after repair
The crankshaft position variation learn not carried out after work on the engine. Misfire detection works by measuring tiny fluctuations in crankshaft speed, and no reluctor wheel is perfectly machined — so the module learns each wheel's own error and subtracts it.
Without that correction the machining tolerance reads as a misfire, usually on the same cylinder every time, on an engine that is mechanically perfect. It is required after replacing the crankshaft, the reluctor, the sensor or the module, and often after any repair that disturbs the relationship. Clearing the codes does not do it: the learned values persist, so the fault returns within a few drive cycles and looks intermittent. The procedure is a specific commanded routine under defined conditions, usually a deceleration from a set speed.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
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