P0AA4: Hybrid/EV Battery Negative Contactor Circuit Stuck Closed
What to do
As P0AA1 — attend to it promptly, accept the car's refusal to restart as the safety system working, and make sure any workshop touching the high-voltage system knows this code is stored.
What this code means
The negative contactor stayed closed when commanded open — P0AA1's weld on the other pole, found the same way: opened at shutdown, voltage watched, contact caught welded.
The same conclusion follows: the pack still isolates through the positive contactor, the car may decline to restart rather than run on one lock, and the urgency is about restoring the second layer of protection rather than any immediate danger. The service-hazard note carries too — anyone working on the system needs to know.
Likely causes
Start here
- Welded battery contactor · specialist
Less common
- Wiring shorted to voltage · moderate
- Fault stored in the battery energy control module · 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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Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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