Cause
Incorrect sensor fitted to bank
A sensor of the wrong type fitted to one bank of a V engine, so the two banks are measured differently. The engine has two of nearly everything and they are not always the same part number — upstream and downstream oxygen sensors differ, and left and right banks sometimes do too.
The module compares banks against each other, so a mismatch produces correlation and rationality codes while each sensor individually reads a plausible value. Every single-sensor test passes, which is what makes this survive a full diagnostic, and the fault only exists in the comparison.
Check the part numbers on both banks against each other and against the catalogue. A wideband sensor fitted where a narrowband belongs, or a heater of a different rating, will fit the same connector and thread. Suspect it after any sensor replacement, particularly where an aftermarket universal sensor has been fitted and spliced — that is the commonest route to two banks that no longer speak the same language.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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