Contacteur de batterie soudé

One of the high-voltage contactors connecting the traction battery to the rest of the car has fused shut and can no longer open.

A contactor is a very large relay, and it is the only thing that disconnects the pack. Turn the car off, crash it, or trip any safety interlock, and these are what open — de-energising everything downstream. The whole high-voltage safety architecture rests on them being able to.

They weld for a physical reason worth understanding, because it explains why precharge exists. An inverter contains substantial capacitors, and closing a contactor onto empty capacitors draws an enormous inrush current for a few milliseconds — enough to melt and fuse the contact faces. So the car first charges those capacitors gently through a resistor, and only then closes the main contactor onto a circuit already at voltage. A failed precharge resistor therefore does not merely log a code: it destroys the contactor it was protecting, and the two faults arrive together.

Why this matters more than most electrical faults is what it takes away. A welded contactor means the pack cannot be isolated — the vehicle is live downstream whether or not it is switched on, and it will still be live after a collision. A car reporting this should not be worked on by anyone without high-voltage training, and should not be assumed safe because it is switched off. The car itself will normally refuse to operate, which is correct.

The usual detection is a self-test at shutdown: the module opens the contactors and checks that the voltage downstream actually falls. One that does not is welded.