Capteur de température du catalyseur d'ozone défectueux

The sensor reporting the temperature of the catalytic coating on the radiator that converts ozone in passing air into ordinary oxygen. The conversion only works within a temperature band, so the module needs this reading before it can credit the conversion or run the monitor that checks the system.

Nothing about the engine depends on it and the driver will notice nothing at all. What is at stake is an emissions monitor that cannot complete — which matters only where that is tested, but matters absolutely there, since a vehicle running perfectly can be unable to take a test.

Its position is the whole diagnostic story. It sits at the very front of the vehicle, in the path of road spray, salt, grit and stone chips, so corroded connectors and damaged wiring vastly outnumber genuine sensor failures. Two checks come before the part: anything that required the front of the car to come apart has had this connector apart — a radiator, a condenser, a bumper, an accident repair — and it is small, hidden and easy to leave unmated; and a fault that follows the weather is water in the plug, not a failing sensor.

The cross-check that finds a sensor genuinely lying is to compare it against the ambient air temperature sensor after the vehicle has stood overnight, when everything at the front has had all night to reach the same temperature and ought to agree.