Cause
Faulty crankcase pressure sensor
The sensor monitoring pressure in the crankcase, faulty so the module cannot tell whether ventilation is working. The sensor exists to detect a disconnected or blocked breather — modern emissions rules require the system to prove its crankcase is sealed and ventilated, and this is how.
A fault here means the check cannot run rather than the engine running badly. The monitor fails, a code is stored, and driveability is usually untouched — which is what distinguishes it from the disconnection it is designed to detect, where the engine idles badly and runs lean.
The reading is easy to misjudge because the pressures are tiny. Crankcase pressure sits within a few millibars of atmospheric, so a small offset is a large proportional error, and a sensor reading plausibly can still be useless. Compare it against atmospheric with the engine stopped — they should agree almost exactly, and a difference there is an offset fault no circuit test will find. Check the sensor's port and hose for oil, which is the commonest cause of a slow or damped response.
Typically a moderate job.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
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