تلوث بالزيت أو سائل التبريد على طرف الحساس

A sensor's tip coated with oil or coolant, insulating it from what it measures. The reading becomes slow rather than wrong, which is the important distinction: it looks plausible at steady state and lags whenever conditions change.

So correlation checks against a neighbouring sensor trip while each value alone seems reasonable. Where the contamination came from matters more than the sensor. Oil means a turbocharger seal, a failing breather or valve stem seals; coolant means an internal leak, and coolant on an exhaust sensor is a head gasket question rather than a sensor one. Inspect the tip on removal — the coating is visible and conclusive — and a new sensor into the same contamination is fouled again within weeks.