P0AA4: Hybrid/EV Battery Negative Contactor Circuit Stuck Closed

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.

High severity. Can I keep driving?If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

Symptoms

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

Most likely causes

Negative contactor contacts welded after high current or arcing
Battery or hybrid controller software (GM Volt 2019, Jeep 4xe updates)
Faulty battery disconnect unit (GM Bolt EV bulletin)
Negative bus voltage sensing error

What to check, in order

Start with the simplest. You can tick off each step.

Read the codes with a hybrid-compatible scanner and write down the freeze frame data
Use the scanner to command the movement and compare the actual position with the requested one.
Make sure the battery cooling air intake is clean
Check the 12 V auxiliary battery, which can also trigger warnings
Leave any high-voltage measurements to a specialized repair shop

Good to know

It can trigger limp mode.
It affects emissions.

What each character of P0AA4 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
A: Hybrid or air and fuel
A4: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0AA4?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code P0AA4?

The most common causes are: negative contactor contacts welded after high current or arcing; battery or hybrid controller software (GM Volt 2019, Jeep 4xe updates); faulty battery disconnect unit (GM Bolt EV bulletin).

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.