P0AA4Hybrid/EV Battery Negative Contactor Circuit Stuck Closed
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- P0AA4 reports that the negative main contactor still conducts while it should be open, usually through welded contacts, so the pack is no longer fully isolated on the negative side. GM tests it once all contactors have been commanded open for more than 120 s, or the bus has already decayed below 9 % of pack voltage: over 30 V on the negative bus in 4 of 6 samples at 12.5 ms sets Type B (two trips, 19OBDG02). GM's 2017 Bolt EV bulletin prescribes contactor cycling and then a new battery disconnect unit; the 2019 Volt update links welded contactors to P0AA1, P0AA4 and P0C76. Jeep cures false sets on the 2023 Grand Cherokee 4xe by software, and Mitsubishi calls it main contactor (-) seizure on the i-MiEV.
- What you may notice
- Vehicle will not start or switch into ready mode
- Check engine light and high voltage system message
- Often stored together with P0AA1 or the slow discharge code P0C76
- Likely causes
- LikelyNegative contactor contacts welded after high current or arcing
- PossibleBattery or hybrid controller software (GM Volt 2019, Jeep 4xe updates)
- PossibleFaulty battery disconnect unit (GM Bolt EV bulletin)
- RareNegative bus voltage sensing error
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €80 – €3,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.