Cause
Worn valve stem seals
The seals controlling how much oil reaches the valve stems, hardened so oil drains into the cylinder while the engine stands. They are a wear-out item rather than a defect, and their failure is one of the few that is diagnosable from the driveway.
The signature is a puff of blue smoke on start-up that clears within seconds and does not return while driving — oil has pooled on the valve overnight and burnt off at once. Worn rings behave differently, smoking under load and continuously. The consequence reaches the diagnostic systems: oil burnt on start-up fouls the plugs and coats the oxygen sensors and catalyst, so misfire and efficiency codes follow. A cold-start misfire on the same cylinder with clean running afterwards is worth taking here rather than to the coil.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
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