Faulty or worn negative contactor (relay) in the high-voltage battery pack that is not closing; Low 12 V battery or poor 12 V supply and earth, so the contactor coil does not receive enough voltage to close; Wiring damage, corrosion or loose connector in the contactor control or feedback circuit; Blown fuse or open circuit in the contactor coil supply; Fault in the battery management or battery control unit that drives the contactor; Open high-voltage interlock or service disconnect not properly fitted, preventing the contactors from closing; Water ingress or physical damage to the battery pack after an impact or flood.

P0AA5

Hybrid/EV Battery Negative Contactor Circuit Stuck Open.

Generic
Powertrain
Generic code
High severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
A: AreaHybrid propulsion
A5: FaultThe specific fault in that area

What it means

P0AA5 is a generic powertrain code meaning the hybrid or EV battery control unit has detected that the high-voltage battery negative contactor circuit is stuck open. The control unit commanded the contactor closed but monitoring of the contactor feedback, circuit voltage or pre-charge behaviour showed that it had not closed. As a result the high-voltage system cannot connect to the battery and the vehicle will usually have limited or no drive.

Symptoms

Hybrid or EV system warning light on the dashboard
Vehicle will not go into Ready mode or will not move
Reduced power or the car running on the petrol engine only (hybrids)
Malfunction indicator lamp on, often with other high-voltage battery codes
Clicking absent or unusual when the car is switched on
Intermittent starting problems that come and go

Possible causes

Faulty or worn negative contactor (relay) in the high-voltage battery pack that is not closinglikely
Low 12 V battery or poor 12 V supply and earth, so the contactor coil does not receive enough voltage to closelikely
Wiring damage, corrosion or loose connector in the contactor control or feedback circuitpossible
Blown fuse or open circuit in the contactor coil supplypossible
Fault in the battery management or battery control unit that drives the contactorpossible
Open high-voltage interlock or service disconnect not properly fitted, preventing the contactors from closingrare
Water ingress or physical damage to the battery pack after an impact or floodrare

How to diagnose

Read all stored codes and freeze frame data, noting any other high-voltage battery, interlock or pre-charge codes and the conditions when it set
Check the state of the 12 V battery, its terminals and the main earth connections, then recharge or test as needed
Inspect the relevant fuses and visually check accessible wiring and connectors for damage, corrosion or water
Check that the service disconnect and high-voltage interlock are correctly fitted and intact
Use live data to compare commanded and actual contactor status, and check the pack voltage readings if the model allows
Only if trained and qualified for high-voltage work, isolate the system following the manufacturer procedure and test the contactor coil circuit and contactor resistance
Check for technical bulletins or software updates for the specific model, as the procedure and access to the contactor vary by make and battery design

How to fix

Charge or replace a weak 12 V battery and clean or repair poor terminals and earths
Repair or replace damaged wiring, connectors or fuses in the contactor circuit
Replace the faulty negative contactor or the battery pack section that contains it, as specified by the manufacturer
Refit or repair the service disconnect or interlock circuit correctly
Repair or replace the battery control unit if testing confirms it is not driving the contactor, applying any approved software update

Can I keep driving?

High severity

Do not rely on the car: it may not start or may lose drive without warning, and high-voltage faults should only be investigated by someone qualified. If it does run, limit driving to getting it to a workshop.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.