P0A31

What it means

P0A31 means the control unit has found that the signal from the temperature sensor for drive motor 'B' (the second traction motor or motor section on vehicles that have one) is plausible electrically but does not behave as expected. The signal may be stuck, drifting, slow to change, or far from the motor A reading or the coolant temperature when the car has been sitting. The fault sits in the hybrid or electric drive system, and the sensor position and the motor that counts as 'B' depend on the model.

Symptoms

• Warning light on the dash, such as the hybrid or EV system warning or the general fault light
• Reduced power or limp mode, with the motor output limited to protect it
• Cooling fans or the electric coolant pump running harder or more often than normal
• Reduced acceleration or regenerative braking
• Temperature readings in live data that are stuck, jumping or unrealistic
• Intermittent fault that comes and goes with temperature or vibration

Possible causes

• Faulty or drifting motor B temperature sensorlikely
• Damaged, chafed or water-affected wiring between the sensor and the control unitlikely
• Corroded, loose or poorly seated connector pins at the motor or inverterlikely
• Coolant loss, air in the system or a weak coolant pump causing a real temperature difference that the control unit judges implausiblepossible
• Coolant or moisture ingress into the motor or inverter connectorpossible
• Fault in the inverter or motor control unit's sensor input circuitrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data. Note motor temperatures, coolant temperature, vehicle speed and load when the fault set, and look for related motor temperature, inverter or cooling pump codes.
2. Check the coolant level and condition in the drive system circuit (it is often separate from the engine circuit on hybrids), and confirm the electric coolant pump and fans are running when they should.
3. After a long cold soak, compare the motor B temperature with motor A, the inverter temperature and the coolant temperature in live data. They should all be close together. Look for a reading that is stuck, offset or erratic.
4. Only a suitably trained technician should work on or near high-voltage components. Follow the manufacturer's isolation procedure before disconnecting any drive motor or inverter connector.
5. Inspect the motor B sensor wiring and connectors for chafing, heat damage, corrosion, moisture, bent pins and poor terminal tension.
6. With the system safely isolated, check the sensor circuit for continuity, short to earth, short to supply and high resistance. Measure the sensor resistance and compare it with the manufacturer's value for the temperature.
7. Wiggle-test the harness with live data on screen to find intermittent faults.
8. If the wiring and sensor test fine, check the manufacturer's data on the control unit input before condemning the unit, as the procedure varies by model.

How to fix

• Replace the motor B temperature sensor, or the motor assembly if the sensor is not serviced separately on that model
• Repair or replace damaged wiring, and fit a new connector or terminals where the pins are corroded or loose
• Clean and dry the connector, find the source of any moisture or coolant ingress and fix it
• Top up, bleed or refill the drive system cooling circuit as specified, and repair any pump or leak fault found
• Replace or repair the inverter or motor control unit only after the sensor, wiring and cooling system have been proven good
• Clear the codes and road test to confirm the fault does not return and the temperature readings are now plausible

Can I keep driving?

Nearby codes

• P0A30GenericDrive Motor B Temperature Sensor Circuit
• P0A32GenericDrive Motor B Temperature Sensor Circuit Low