Contacts de relais soudés

A relay whose contacts have fused closed, so the circuit it controls stays permanently powered. A relay switching an inductive load draws an arc every time it opens, and that arc erodes and eventually welds the contacts — so this is a wear-out failure of a component nobody treats as a wearing part.

The signature is a device that runs when it should not and cannot be switched off, including with the ignition off — a fuel pump that keeps running, a fan that will not stop, a heater that stays on. A component that runs continuously and flattens the battery overnight is a welded relay far more often than it is a module commanding it.

The module detects it by monitoring the circuit it thinks it switched off. Seeing voltage present when it commanded none, it reports a control circuit fault — naming the circuit rather than the relay. Swap the relay with an identical one elsewhere in the box as a first test, which is free and definitive. Ask what welded it: a shorted or stalling motor draws current well beyond the relay's rating, so the new relay welds too unless the load is checked.