Sensor incorrectly positioned after repair

A sensor refitted in the wrong place, at the wrong depth, or facing the wrong way, so its reading is honest about a position that is not the one the module assumes. Every individual test on it passes, because nothing is electrically or mechanically wrong — which is why this fault so often survives a full diagnostic.

Correlation codes are its natural home. Where a system carries several sensors in different positions, the module compares them against each other, so a sensor sitting where its neighbour should be produces a rationality failure while each reading, taken alone, is entirely reasonable. A correlation code on a system that was recently opened is this cause until ruled out.

Depth and orientation matter as much as location. A particulate or temperature sensor not inserted to its shoulder sits outside the flow it should be sampling and responds slowly; one rotated away from the stream reads the pipe wall. Compare the reading's behaviour against a known-good position rather than its absolute value — the tell is a sensor that responds sluggishly or barely moves where its sibling moves sharply.