تبديل موصلات حساسات الأكسجين أثناء الإصلاح

The upstream and downstream oxygen sensors plugged into each other's connectors. The two often carry identical plugs, sit within reach of one another and get replaced as a pair, which is precisely the situation in which they go back the wrong way round.

The module detects it from the shape of the signals rather than from any identification. A pre-catalyst sensor swings rapidly as fuelling is trimmed; a post-catalyst sensor should sit almost flat. Seeing the flat trace where it expects movement, and movement where it expects calm, leaves only one explanation.

The consequence is worse than a stored code, because the engine is now being fuelled from a sensor reading gas that has already passed through the converter. The mixture drifts, the converter runs hot, and catalyst efficiency codes follow. Two new sensors fitted the same crossed way set the same code, which is how this occasionally survives a repair.