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An oxygen sensor whose response has slowed with age rather than failed outright. This is honest wear: the sensor lives at several hundred degrees in a corrosive gas stream and switches millions of times, and its transitions get gradually lazier over tens of thousands of miles.

The distinction that decides the repair is slow versus delayed. Slow response is about the rate of the transition — it starts when it should and takes too long to get there, which is ageing. Delayed response is dead time — it does not begin moving until well after the mixture changed, which is usually an exhaust leak, a sensor out of the gas stream, or a fouled shield. Ageing does not produce dead time, so a delayed-response code on a low-mileage sensor is asking a different question.

Position decides urgency. An upstream sensor controls the fuelling, so a lazy one makes the mixture swing wider than intended and costs economy continuously. A downstream sensor only judges the catalytic converter, so a lazy one blocks emissions monitoring while the car drives normally.

Premature ageing is a symptom, not a verdict. A sensor lazy well before its time was poisoned by something — oil, coolant, or silicone from non-approved sealant — and a replacement fitted into the same exhaust meets the same contamination.