P0AA5: Hybrid/EV Battery Negative Contactor Circuit Stuck Open

P0AA5 is set when the negative main contactor stays open or fails to conduct after the controller has commanded it closed, so the negative side of the high-voltage bus never connects. Audi reports it, alone or with P0AA2, on the 2020-2022 Q5 e quattro and 2021-2022 A7 e quattro plug-in hybrids with a no-start complaint; the stated cause is oxidation of the contact surfaces in the battery junction box after very long standing or extreme humidity. Audi's procedure: several ignition cycles and an EV road test with many consumers on, and a new junction box only if the fault returns. GM includes P0AA5 among the Ultium EV high-voltage codes that require pack data retrieval. It is the counterpart of P0AA4 (welded contacts).

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 does not start or reach ready mode
Hybrid or high voltage battery warning
Charging not possible

Most likely causes

Oxidised contact surfaces after long standing or high humidity (Audi)
Failed battery junction box or burnt negative contactor
Contactor coil not energised due to wiring or 12 V supply fault

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
Check the continuity of the wire between the component and the control module.
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 P0AA5 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
A: Hybrid or air and fuel
A5: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0AA5?

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 P0AA5?

The most common causes are: oxidised contact surfaces after long standing or high humidity (Audi); failed battery junction box or burnt negative contactor; contactor coil not energised due to wiring or 12 V supply fault.

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.