P0A40 means the ECU has detected a **fault in the Drive Motor “A” temperature sensor circuit**, the sensor responsible for reporting how hot the main electric motor gets during operation. If the signal from this thermistor-based sensor is missing, incorrect, or out of range, the ECU can’t monitor motor temperature accurately. This leads to torque reduction, EV mode dropout, or full system shutdown to prevent overheating. In this guide, you’ll learn what P0A40 means, the symptoms it causes, the likely failures, how to diagnose it step-by-step, and the repairs that fix the issue correctly.
What Does P0A40 Mean?
P0A40 sets when the Hybrid/EV Powertrain Control Module sees a **temperature signal from Drive Motor “A” that is invalid or electrically incorrect**. This sensor is typically an NTC thermistor embedded in the stator or housing of the main traction motor. When its resistance, voltage, or signal pattern falls outside the expected range, the ECU assumes the motor could overheat unseen — triggering protection mode.
Because traction motors generate extreme heat under load, the ECU immediately limits or disables electric drive if the sensor signal becomes unreliable.
Quick Reference
- OBD-II Family: P-Code (Hybrid/EV Powertrain)
- Scope: Generic
- System: Drive Motor “A” Temperature Monitoring
- Difficulty Level: Moderate
- Estimated Repair Cost: €120–€500
- Last Updated: 2025-11-24
Real-World Example / Field Notes
P0A40 appears on Toyota/Lexus hybrids, Hyundai/Kia EVs, GM/Voltec vehicles, and Nissan e-Power systems. In a Toyota Prius example, the motor temperature readings became stuck at –40°C (default for an open circuit), causing the engine to run constantly and EV torque to drop sharply. The cause was a broken wire inside the harness near the inverter. On a Hyundai Ioniq EV, the sensor intermittently read 140°C without actual overheating — traced to a coolant leak that wicked into the sensor connector and altered resistance.
Symptoms of P0A40
- Reduced EV torque: System limits power to prevent overheating.
- EV mode dropout: Hybrids may prioritize the gasoline engine.
- Incorrect temperature readings: Often –40°C (open) or 150°C+ (short).
- Limp mode: Vehicle may restrict acceleration or top speed.
- Warning messages: “Check Hybrid System,” “EV System Error,” or “Service HV System.”
- Possible overheating behavior: Cooling pump and fans may run at max speed.
- READY mode fail: Severe or persistent faults may prevent startup.
Common Causes of P0A40
Most Common Causes
- Open circuit in the motor temperature sensor wiring.
- Short-to-ground or short-to-power in the signal line.
- Failed NTC thermistor in the drive motor.
- Corroded or contaminated temperature sensor connector.
- Harness damage from vibration, rubbing, or road debris.
Less Common Causes
- Coolant intrusion affecting sensor resistance values.
- Internal motor stator temperature-sensing circuit failure.
- Incorrect readings caused by inverter cooling issues.
- ECU input driver malfunction (rare).
Diagnosis: Step-by-Step Guide
The goal is to determine whether the temperature signal is incorrect due to wiring, the sensor, or internal motor electronics.
Tools You’ll Need: EV-safe scan tool, multimeter, wiring diagram, IR thermometer, cooling-system tools, and oscilloscope (optional).
- Monitor live temperature data. Look for fixed values such as –40°C (open circuit) or 150°C (short).
- Inspect the sensor connector. Check for corrosion, pushed-out pins, or coolant residue.
- Perform a wiggle test. Move the harness near the inverter and motor; flickering temperatures indicate wiring faults.
- Check sensor resistance. Compare resistance to specifications at ambient temperature.
- Test continuity in the signal wire. Open or high-resistance circuits cause unstable readings.
- Inspect wiring near the motor. Areas near the subframe or under the vehicle are prone to damage.
- Confirm real motor temperature. Use an IR thermometer to check if the motor is actually heating abnormally.
- Check inverter coolant flow. Cooling issues can cause real overheating and abnormal temperature swings.
- Verify ECU input voltages. If wiring and sensor test good, suspect ECU input fault.
- Review freeze-frame data. Conditions at the time of the fault help pinpoint load-related failures.
Pro Tip: If the temperature reading is exactly –40°C at all times, the circuit is fully open — start by inspecting wiring and connector integrity before replacing the sensor or motor assembly.
Possible Fixes & Repair Costs
- Repair open or shorted wiring: €10–€100.
- Clean or replace corroded connectors: €10–€50.
- Replace Drive Motor “A” temperature sensor (if serviceable): €120–€250.
- Fix coolant leaks around the sensor/harness: €40–€150.
- Repair or replace drive motor stator sensor circuit: €300–€500+ (model-dependent).
- Bleed and restore cooling flow: €40–€120.
Before replacing expensive components like the motor assembly, always confirm wiring integrity and correct sensor resistance.
Can I Still Drive With P0A40?
Driving may be possible, but it’s risky. Incorrect motor temperature readings can cause sudden torque reduction or system shutdown. If the ECU cannot verify actual temperature, it will prioritize safety — often disabling EV power. If temperature spikes or warnings appear, stop driving and diagnose the issue immediately.
Related Codes
- P0A6C – Hybrid Battery Voltage System Isolation Sensor Range/Performance
- P0A9F – Hybrid Battery Temperature Sensor Too Hot
- P0A9E – Hybrid Battery Temperature Sensor Too Cold
- P0A9D – Hybrid Battery Temperature Sensor Range/Performance
- P0A9C – Motor Electronics Coolant Temperature Sensor Circuit High
- P0A9B – Motor Electronics Coolant Temperature Sensor Circuit Low
- P0A9A – Motor Electronics Coolant Temperature Sensor Circuit Range/Performance
- P0A99 – Motor Electronics Coolant Temperature Sensor Circuit
- P0A98 – Hybrid Battery Pack Cooling Fan 2 Performance
- P0A97 – Hybrid Battery Pack Cooling Fan 2 Control Circuit
Key Takeaways
- P0A40 indicates an electrical fault in Drive Motor “A” temperature sensing.
- Most cases involve open circuits, wiring faults, or failed thermistors.
- Incorrect temperature data forces torque reduction or limp mode.
- Live data and resistance testing are the fastest ways to pinpoint the issue.
FAQ
What causes P0A40 most often?
Broken wiring, open circuits, or a failed temperature sensor are the most common causes of this code.
Why does the temperature show –40°C?
–40°C is the default reading for an open thermistor circuit indicating no valid temperature signal.
Is P0A40 dangerous?
Potentially. If the ECU cannot trust motor temperature, it may disable EV power to prevent overheating damage.
How do I diagnose P0A40?
Use live data, inspect wiring and connectors, check sensor resistance, confirm coolant flow, and compare readings with IR thermometer values.
Can I drive with P0A40?
Short trips may be possible, but torque loss or limp mode can occur. Repair the circuit promptly to protect the drive motor.