P0A08 means the ECU has detected a **high circuit voltage** condition in the Drive Motor “A” Temperature Sensor — the sensor responsible for monitoring the temperature of the main electric drive motor in a hybrid or fully electric vehicle. A “circuit high” fault usually happens when the ECU sees voltage that’s too high to be realistic, which is typically caused by an open circuit, disconnected connector, broken wire, or an internally open thermistor. When this occurs, the system may reduce electric power or enter fail-safe mode to protect the motor. This guide explains what P0A08 means, the symptoms, causes, diagnostic steps, and the typical fixes for this motor temperature sensor circuit fault.
What Does P0A08 Mean?
P0A08 sets when the ECU detects that the Drive Motor “A” temperature sensor signal is **above the normal expected range**. Drive motor temperature sensors use NTC thermistors, meaning resistance increases when the sensor is cold. If the circuit becomes open or the connector disconnects, the voltage will rise to maximum — causing the ECU to interpret the temperature as extremely cold or invalid.
Since the ECU cannot trust the sensor reading, it may reduce motor torque, disable EV mode, or activate protective cooling strategies to prevent motor overheating.
Quick Reference
- OBD-II Family: P-Code (Hybrid/EV Powertrain)
- Scope: Generic
- System: Drive Motor Temperature Monitoring
- Difficulty Level: Moderate
- Estimated Repair Cost: €80–€300
- Last Updated: 2025-11-24
Real-World Example / Field Notes
P0A08 frequently appears on Toyota Prius, Lexus hybrids, Hyundai/Kia hybrids, Chevrolet Volt, and many EVs including the Nissan Leaf. In one Toyota Prius case, the ECU reported the drive motor temperature as –40°C the moment the car powered on. This is a classic sign of an open circuit. The cause was a cracked connector that lost contact whenever the vehicle vibrated. Replacing the connector pigtail solved the fault immediately. On Nissan EVs, corrosion inside the sensor connector caused by road salt commonly triggers P0A08.
Symptoms of P0A08
- Reduced EV power: ECU limits torque to prevent overheating.
- Limp mode: Hybrid system may switch to gasoline engine primarily.
- Warning lights: “EV System Error,” “Check Hybrid System,” or “Motor Overheating.”
- Cooling fan activation: ECU compensates for missing temperature data.
- Loss of smooth EV operation: EV mode may be blocked.
- Unrealistic temperature readings: Often –40°C or extremely low values.
- Check Engine Light: P0A08 often shows up with inverter or cooling system codes.
Common Causes of P0A08
Most Common Causes
- Open circuit in the temperature sensor wiring.
- Disconnected or loose sensor connector.
- Failed Drive Motor “A” temperature sensor (internal open circuit).
- Corroded sensor terminals causing high resistance.
- Broken wiring near the motor or inverter.
Less Common Causes
- Low coolant exposing the sensor (liquid-cooled motors).
- Improper coolant or contamination affecting thermal transfer.
- ECU internal signal interpretation fault (rare).
- Sensor mounting failure causing poor thermal contact.
- Air pockets in the cooling jacket creating temporary open-circuit behavior.
Diagnosis: Step-by-Step Guide
The goal is to determine whether the high voltage signal is due to an open circuit, connector issue, wiring damage, or a failed sensor.
Tools You’ll Need: Hybrid/EV-capable OBD-II scanner, multimeter, IR thermometer, wiring diagram, coolant service tools (for liquid-cooled motors).
- Check live data. A reading of –40°C or similarly low values is classic for an open circuit.
- Inspect the connector. Look for loose pins, broken clips, corrosion, or moisture inside the housing.
- Perform a wiggle test. Move the harness while monitoring live data. Sudden temperature jumps mean a bad connection.
- Check wiring continuity. Measure resistance from the sensor connector back to the ECU. Infinite resistance confirms an open.
- Test sensor resistance. Compare to manufacturer temp/resistance charts. Infinite resistance indicates internal failure.
- Check coolant level and condition. Low coolant can affect temperature sensor contact and readings.
- Use an IR thermometer. Compare actual motor housing temperature to the scanner reading.
- Inspect wiring near hot components. Look for melted, cracked, or heat-soaked insulation.
- Look for related codes. P0A2F or P0A94 may point to broader thermal control issues.
Pro Tip: If the temperature reading instantly jumps from normal to –40°C when the harness is flexed, you’ve found an open-circuit connection — usually a broken wire or cracked pin.
Possible Fixes & Repair Costs
- Repair or replace broken wiring: €10–€80.
- Replace Drive Motor “A” Temperature Sensor: €80–€200.
- Clean or replace corroded connector terminals: €10–€40.
- Fix coolant leaks near the connector (if applicable): €20–€120.
- Bleed or refill cooling system: €40–€120.
- Replace coolant pump (liquid-cooled motors): €100–€350.
Because P0A08 is usually an open-circuit issue, wiring and connector inspection should always come before sensor replacement.
Can I Still Drive With P0A08?
Driving with P0A08 is risky because the ECU cannot determine the true motor temperature. It may disable or severely limit electric drive unexpectedly. While the vehicle may still move, overheating or sudden power loss is possible. It’s best to avoid long trips until the fault is repaired.
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
- P0A08 indicates a high-voltage (open circuit) fault in Drive Motor “A” temperature sensor circuit.
- Most causes involve loose connectors, broken wires, or a failed sensor.
- Driving may be possible but risks overheating and sudden loss of EV power.
- Live data and continuity testing quickly reveal open-circuit conditions.
FAQ
What causes P0A08 most often?
A broken wire, cracked connector, or failed sensor is the most common cause of a high-voltage reading.
Why does the temperature show –40°C?
–40°C is a default value when the ECU detects an open circuit — the sensor signal is missing entirely.
Can coolant issues cause P0A08?
Yes, on liquid-cooled motors. Low coolant or air pockets can cause poor thermal contact, though true P0A08 is usually electrical.
How do I diagnose P0A08 accurately?
Inspect and test wiring, compare live data to IR thermometer values, check continuity, and verify sensor resistance.
Can I still drive with P0A08?
Short distances may be possible, but sudden power reduction or EV shutdown is likely. Repair the issue as soon as possible.