Fuite d'échappement en amont de la sonde

A blowing joint, cracked manifold or corroded pipe ahead of the sensor, letting air into the exhaust stream before the sensor measures it. The sensor is then reading a diluted sample and reporting it faithfully — which is why replacing it changes nothing.

It matters because exhaust pulses are not a steady flow. Between pulses the pressure at a leak can drop below atmospheric and draw fresh air in, so a leak admits oxygen even though the exhaust is nominally under pressure. Oxygen, NOx, temperature and soot sensors are all misled by it, each in its own direction.

The audible clue is a ticking or blowing noise that is loudest from cold and quietens as the metal expands and closes the gap — which is exactly why it is so often missed on a warm car on a ramp. On an untouched vehicle setting a sensor correlation or sample-error code, this is the first thing to rule out.