P0C77: Hybrid/EV Battery System Precharge Time Too Short
What to do
Treat this as a safety-relevant fault rather than a timing nuisance. One of its readings is that a contactor did not open when the car was last shut down, which means the high-voltage system may be live with the car apparently off. Nobody without high-voltage training should be working near this vehicle until it has been checked, and it should not be dismissed because the car still drives.
What this code means
The high-voltage system's precharge took less time than it should have.
Precharge is a two-second ritual the car performs every time it powers up, and it exists to protect the contactors. The inverter contains large capacitors, and closing a main contactor onto empty capacitors draws an enormous inrush for a few milliseconds — enough to weld the contact faces together. So the car first charges those capacitors gently through a resistor, watches the voltage rise, and only closes the main contactor once both sides are nearly equal.
The time that takes is a measurement, and the module treats it as one. Capacitance and resistance are both known, so the expected duration is known. Too short means the capacitors reached voltage faster than that much capacitance should allow through that much resistance — and the readings that produce it are: the precharge resistor being bypassed or shorted, a capacitor bank that has lost capacitance with age, or a contactor already closed when the sequence began, which is the one that matters, because it means the pack was never actually isolated.
That last reading is why this is not a minor timing complaint. A car that cannot isolate its own pack is a car that is live when it should not be.
Likely causes
Start here
- Welded battery contactor · specialist
- Failed precharge resistor · specialist
Less common
- Faulty battery contactor · specialist
- Faulty power inverter · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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