P0A42 means the ECU has detected a **low-voltage condition in the Drive Motor “A” temperature sensor circuit**, which monitors how hot the main traction motor gets during operation. “Circuit Low” means the sensor signal is reading lower than expected — usually caused by a short-to-ground, internal thermistor failure, or wiring damage. When the ECU can’t trust this temperature reading, it limits torque, drops EV mode, or enters limp mode to protect the motor from potential overheating. This guide explains what P0A42 means, what causes it, how to diagnose it, and the repairs that typically fix this fault.
What Does P0A42 Mean?
P0A42 sets when the Hybrid/EV Powertrain Control Module detects that the **voltage from the Drive Motor “A” temperature sensor is below the valid operating range**. These sensors use NTC thermistors, which naturally lower resistance as temperature rises. But if the voltage drops too low — lower than any real motor temperature would produce — the ECU assumes the circuit has a short or the sensor has failed.
Because traction motors can overheat quickly under load, the ECU responds immediately by reducing torque or shutting down EV power to prevent stator damage.
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–€400
- Last Updated: 2025-11-24
Real-World Example / Field Notes
This code shows up frequently on Toyota/Lexus hybrids, Hyundai/Kia EVs, and GM/Voltec platforms. In a Toyota RAV4 Hybrid case, P0A42 appeared after heavy rain. The sensor connector had filled with water, shorting the signal line to ground and causing a fixed high temperature reading of 150°C. In a Hyundai Ioniq Electric, the internal thermistor inside the motor failed, pulling the circuit low only during high-load warmup — a tricky intermittent short that took live-data graphing to confirm.
Symptoms of P0A42
- Fixed high temperature reading: Often 140–150°C regardless of load.
- Reduced EV power: The ECU limits torque to protect the motor.
- EV mode dropout: Hybrid systems may rely on the engine instead.
- Limp mode activation: Acceleration may become severely restricted.
- Warning lights: “Check Hybrid System,” “EV System Error,” or similar alerts.
- Cooling system running hard: Pump and fans may stay at max even when motor is cool.
- READY mode failure: In extreme cases, the vehicle may not start in EV mode.
Common Causes of P0A42
Most Common Causes
- Short-to-ground in the temperature sensor signal wire.
- Failed NTC thermistor inside the drive motor.
- Water or coolant intrusion inside the sensor connector.
- Chafed or pinched wiring near the inverter/motor assembly.
- Sensor connector contamination causing low-voltage path.
Less Common Causes
- Shorted harness segments due to corrosion or road salt.
- Internal stator temperature-sense circuit failure.
- ECU driver circuit reading ground reference incorrectly (rare).
- Cooling system restrictions causing real temperature spikes (but these rarely cause P0A42 alone).
Diagnosis: Step-by-Step Guide
Your goal is to confirm whether the low-voltage condition is caused by wiring damage, a shorted sensor, or internal motor failure.
Tools You’ll Need: EV-safe scan tool, multimeter, wiring diagram, IR thermometer, coolant tools, and optionally an oscilloscope for signal tracing.
- Check live data. A constant very-high temperature reading (140°C–150°C) usually indicates a short-to-ground.
- Inspect the sensor connector. Look for moisture, corrosion, bent pins, or coolant residue.
- Perform a wiggle test. Move the harness with live data visible — sudden spikes confirm wiring shorts.
- Measure signal voltage. Very low voltage at the ECM or sensor connector confirms a grounded circuit.
- Check sensor resistance. A failed thermistor will show very low resistance at room temperature.
- Test continuity to ground. If the signal wire shows continuity to ground, trace the harness for damage.
- Inspect wiring under the vehicle. Damage is common where the harness touches the subframe.
- Confirm actual motor temperature. Use an IR thermometer; if the motor is cool, the reading is false.
- Check cooling flow. A real overheating condition won’t cause P0A42 but may accompany other codes like P0A94.
- Review freeze-frame data. Helps determine whether the failure occurs on startup, under load, or during regen braking.
Pro Tip: If temperature instantly jumps to maximum (140–150°C) the moment the vehicle enters READY mode, the sensor signal is shorted to ground — this is the most common pattern for P0A42.
Possible Fixes & Repair Costs
- Repair shorted wiring or insulation damage: €10–€100.
- Clean or replace contaminated connectors: €10–€50.
- Replace Drive Motor “A” temperature sensor (if serviceable): €150–€300.
- Repair or replace internal motor temperature circuit: €300–€500+ (motor-dependent).
- Fix coolant leaks causing connector contamination: €40–€150.
- Bleed and restore inverter/motor cooling flow: €40–€120.
Do not replace the motor or major components until you confirm the signal wire isn’t grounded or damaged — wiring faults are far more common.
Can I Still Drive With P0A42?
Driving is possible but risky. A shorted temperature sensor signal forces the ECU to assume the motor is overheating, which can cause sudden torque reduction or limp mode at any time. If EV power drops unexpectedly or overheating warnings appear, stop driving and repair the circuit 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
- P0A42 indicates a low-voltage fault in Drive Motor “A” temperature sensing.
- Most causes involve shorts-to-ground, wiring damage, or sensor contamination.
- The ECU may cut torque suddenly to protect the motor.
- Fixed high-temperature readings are the most important diagnostic clue.
FAQ
What causes P0A42 most often?
A short-to-ground in the sensor signal wire or a failed thermistor inside the motor.
Why does the temperature show extremely high values?
Very high readings (140–150°C) typically occur when the sensor circuit is grounded, creating artificially low voltage.
Is P0A42 dangerous?
Yes. The ECU may limit power or shut down EV propulsion to avoid potential overheating.
How do I diagnose P0A42?
Check live data, inspect wiring and connectors, test resistance, and verify that the signal wire is not grounded.
Can I drive with P0A42?
It’s possible but risky. Sudden limp mode or torque loss can occur at any time. Repair the circuit promptly.