P0AC8
Hybrid/EV Battery Temperature Sensor B Circuit High.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- A
- AreaHybrid propulsion
- C8
- FaultThe specific fault in that area
What it means
P0AC8 means the hybrid or EV battery control unit has seen the signal from battery temperature sensor B sitting higher than its valid range. A high reading on a thermistor-type sensor usually means an open circuit, a poor connection or a short to a voltage supply, rather than a genuinely hot battery. The control unit can no longer trust the temperature data for that part of the pack, so it may restrict battery use to protect it.
Symptoms
- Hybrid or EV system warning light on, often with the main malfunction indicator
- Reduced power or limited electric assist
- Reduced regenerative braking or limited charging
- Battery cooling fan running constantly or at an odd speed
- Poor fuel economy or reduced electric-only range
- Hybrid system refusing to enter Ready mode in some cases
Possible causes
- Open circuit or high resistance in the sensor wiring or connector, such as a corroded, loose or pushed-back terminallikely
- Faulty battery temperature sensor B, with an internal open circuitlikely
- Short to voltage in the sensor signal wire, caused by damaged insulation or water ingresspossible
- Open or poor sensor ground connectionpossible
- Fault in the battery monitoring or battery control unit, including its sensor input circuitrare
- Damage to the battery pack harness or sensor from previous repair or impact workrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting any other battery temperature or cooling fan codes
- Compare live data from all battery temperature sensors; sensor B reading an extreme or fixed value while the others are normal points to a circuit fault
- Visually inspect accessible harness and connectors for damage, corrosion, moisture or loose terminals, only after the high-voltage system has been made safe by a suitably trained technician
- Check the battery cooling ducts, fan and air intake for blockages to rule out a real temperature problem
- With the system isolated, check the sensor resistance against the manufacturer's specification at a known temperature; the correct procedure depends on the model
- Check the sensor signal, reference voltage and ground wiring for continuity and for shorts to voltage, following the wiring diagram for that vehicle
- If wiring and sensor are good, check for a relevant software update and test the battery control unit input as the manufacturer specifies
- Clear the code and road test, then confirm it does not return and that the sensor readings track each other sensibly
How to fix
- Repair or replace the damaged wiring, and clean or replace faulty connector terminals
- Replace the faulty battery temperature sensor or sensor assembly in line with the manufacturer's procedure
- Repair any short to voltage and fix the cause of water ingress or chafing
- Restore the sensor ground connection
- Replace or reprogram the battery control unit only after confirming the fault is in the unit, and apply any official calibration update that addresses a known fault
Can I keep driving?
The car can usually be driven for a short time, but expect reduced power or limited battery use, and book it in soon. Stop driving if the hybrid or EV system shows a serious warning, there is a burning smell, or the car loses drive.<br>Work on the high-voltage battery and wiring must only be carried out by trained technicians.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.