Failed precharge resistor

The resistor that gently charges the high-voltage capacitors before the main contactor closes has gone open, drifted high in value, or cracked.

Understanding what it is for explains both why it fails and why its failure destroys something more expensive. An inverter contains substantial capacitors, and closing a contactor straight onto empty ones draws an enormous inrush for a few milliseconds — enough to melt and weld the contact faces together. So the car first charges them through a resistor, watches the voltage rise until both sides are nearly equal, and only then closes the main contactor onto a circuit already at voltage.

The resistor absorbs that inrush as heat, every single time the car is switched on. That is its entire job, and it is why they age: repeated thermal cycling cracks the element or drifts its value upward.

Which produces the failure chain worth knowing. A resistor that has gone high stretches the precharge until the module gives up — the car simply will not power up, and the code names the timing rather than the part. A resistor that has gone fully open removes the protection altogether, and the next contactor closure is the one that welds the contactor. So a precharge fault left unattended does not stay a precharge fault; it becomes a welded contactor, which is both more expensive and a genuine safety matter because the pack can then no longer be isolated.

The two therefore arrive together often enough that finding one should prompt checking the other — a welded contactor with no explanation is a precharge circuit that failed first.

It is a small and comparatively cheap component, so it is worth asking whether it was measured before anything larger is quoted. Measuring it is a resistance check by someone with high-voltage training; it is not an owner-serviceable item.