Sonde lambda mal vissée

A sensor that is fitted but not sitting where it needs to sit — not screwed to depth, missing its crush washer, cross-threaded, or rotated so its shield openings face out of the flow. The element has to be in the moving gas stream to measure it, and one sitting in a pocket or recessed behind a boss samples gas that has already passed.

It presents as delayed response rather than as an electrical fault, which is what sends the diagnosis toward a new sensor. The circuit is sound, the element works, and the reading simply arrives late because the gas does.

An imperfect seal is the second half of the same problem. Exhaust pulses drop below atmospheric between beats, so a thread that does not seal draws fresh air in rather than merely blowing gas out — putting oxygen onto the element and skewing the reading toward lean, most visibly on the lean-to-rich transition.

It follows exhaust work almost every time. Ask what was done before ordering a part: the fix is usually the sensor removed, the washer replaced and the thread cleaned, at no parts cost. Anti-seize on the threads matters here too — the wrong compound, or any on the sensor tip, contaminates the element it was meant to protect.