P0AC9
Hybrid/EV Battery Temperature Sensor B Circuit Intermittent/Erratic.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- A
- AreaHybrid propulsion
- C9
- FaultThe specific fault in that area
What it means
P0AC9 is a generic powertrain code meaning the hybrid or EV battery control unit has seen an intermittent or erratic signal from Hybrid/EV Battery Temperature Sensor B. The reading jumps, drops out or changes implausibly quickly compared with the other battery temperature sensors or with expected thermal behaviour. Which physical sensor is 'B' and where it sits in the pack depends on the model.
Symptoms
- Hybrid or EV system warning light, or malfunction indicator lamp, on
- Reduced power or limited electric assist or regeneration
- Battery cooling fan running loudly or constantly
- Poor fuel economy on a hybrid as the engine is used more
- Reduced charging rate or reduced EV range
- Fault appearing and disappearing, with no consistent pattern
Possible causes
- Loose, corroded or damaged connector at the battery temperature sensor or battery control unitlikely
- Damaged or chafed wiring in the sensor circuit causing intermittent open circuit or shortlikely
- Failing battery temperature sensor (thermistor) giving erratic readingslikely
- Moisture or contamination in the pack connectors or sensor harnesspossible
- Fault within the battery control unit or its monitoring circuitsrare
- Outdated control unit software where a manufacturer bulletin covers the faultrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting the temperatures and conditions when the fault set, and check for other battery temperature or cooling codes.
- Check for manufacturer bulletins and software updates relevant to the model and fault.
- Using live data, compare all battery temperature sensor readings at rest; they should be similar after the car has been sitting. Look for one sensor that jumps or drops out.
- Inspect the battery cooling air intake, ducts and fan for blockages or debris, as the fault may be mistaken for a thermal issue.
- Following the manufacturer's high-voltage safety procedure, and only if qualified, visually inspect the sensor wiring and connectors for damage, corrosion, moisture or poor pin tension.
- Wiggle-test the harness and connectors while watching live temperature data to reproduce the intermittent fault.
- Measure sensor resistance and circuit continuity against manufacturer specifications, as values vary by model.
- If wiring and sensor test fine, have the battery control unit checked using the manufacturer's procedure.
How to fix
- Clean, reseat or replace faulty connectors and terminals in the sensor circuit.
- Repair or replace damaged wiring sections in the sensor harness.
- Replace the faulty battery temperature sensor or sensor assembly as the manufacturer specifies.
- Dry out and seal any water ingress and correct the cause of it.
- Update the control unit software if an outdated version is a known cause for the model.
- Replace the battery control unit only after all other causes have been ruled out.
Can I keep driving?
The car can usually be driven short distances, but power may be limited and the battery is not being properly monitored for temperature. Book it in soon, and stop driving if a red hybrid warning appears, power is lost or there is a burning smell.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.