P0A91 – Meaning, Symptoms, Causes & How to Fix It

P0A91 – Drive Motor “B” Performance is a hybrid and electric vehicle trouble code that appears when the powertrain control module detects abnormal performance from Drive Motor B. This can lead to reduced power, warning lights on the dash, or the vehicle entering a protective mode designed to prevent high-voltage damage. Because Drive Motor B plays a critical role in propulsion and regenerative braking, ignoring this code can increase wear on major components. This guide explains exactly what P0A91 means, what causes it, how to diagnose it, and which repairs actually fix the problem.

What Does P0A91 Mean?

P0A91 is a generic OBD-II hybrid/EV propulsion code defined under the SAE J2012-DA standard. The ECU sets this code when Drive Motor B delivers less torque than expected or its feedback signals don’t match the commanded output. The control module uses torque sensors, inverter phase current data, resolver (position sensor) readings, and temperature inputs to detect any mismatch.

Common triggers include increased mechanical drag, winding resistance, inverter efficiency loss, or electrical noise that disrupts the motor’s 3-phase waveform. When the ECU identifies abnormal performance, it logs freeze-frame data and may place the vehicle in a limited-power mode to protect the high-voltage system.

Quick Reference

OBD-II Family: P0xxx – Hybrid/EV Propulsion
Scope: Generic (SAE J2012-DA)
System: Drive Motor & Inverter
Difficulty Level: High
Estimated Repair Cost: €250–€4,000+
Last Updated: 2025-12-08

Real-World Example / Field Notes

On Toyota and Lexus hybrid platforms, Drive Motor B typically refers to MG2. I’ve seen P0A91 appear when the vehicle has been driven far beyond recommended transaxle fluid intervals. One memorable case involved a Prius where the owner complained of weak EV acceleration. The inverter tested fine, but mechanical drag inside the transaxle was causing torque deviation. The unexpected culprit was a failing MG2 bearing. After replacing the transaxle assembly, torque output returned to spec and the code disappeared.

Symptoms of P0A91

Check Engine Light: Often paired with hybrid system warnings.
Reduced Power: Noticeable drop in electric assist and acceleration.
Limp Mode: Vehicle may limit speed to prevent high-voltage damage.
Harsh EV Transitions: Hesitation when switching between EV and engine power.
Decreased Efficiency: Higher energy consumption or shorter EV range.
Unusual Noises: Whining or grinding from the transaxle area.
Inverter Overheating: Cooling fans running louder or more frequently.

Common Causes of P0A91

Most Common Causes

Internal damage or drag in Drive Motor B (bearing failure, winding resistance).
Inverter/converter performance loss affecting voltage output.
Faulty resolver (motor position sensor) sending incorrect feedback.
High-voltage connector corrosion or elevated resistance at motor terminals.
Restricted inverter coolant flow causing thermal imbalance.

Less Common Causes

Contaminated or degraded hybrid transaxle fluid increasing mechanical load.
Outdated hybrid ECU firmware affecting torque calculations.
HV battery abnormalities causing fluctuating current delivery.
Planetary gearset binding affecting MG2 rotation.
Inverter coolant pump malfunction reducing cooling efficiency.

Diagnosis: Step-by-Step Guide

Start by confirming the code and reviewing freeze-frame data to see the conditions under which it set.

Tools You’ll Need: Hybrid-capable OBD-II scanner, multimeter, megohmmeter (insulation tester), IR thermometer or thermal camera, oscilloscope (optional), and proper high-voltage safety gear.

Scan for related codes. Look for inverter temperature or MG2 torque deviation codes that help pinpoint the issue.
Check inverter coolant flow. Verify pump operation and eliminate air pockets; overheating reduces motor efficiency.
Compare commanded vs. actual torque. If the mismatch is large, the motor is underperforming.
Inspect HV connectors. Corrosion or loose pins at the inverter and motor terminals increases resistance.
Test motor winding insulation. Megohmmeter readings below specification suggest internal motor failure.
Check resolver feedback. Irregular resolver angle or voltage readings disrupt control logic.
Evaluate inverter output. An oscilloscope can reveal waveform distortion in the 3-phase output.
Check for mechanical drag. With the HV system disabled, rotate the transaxle to detect binding or bearing issues.
Verify ECU software version. Some models require updated torque calculation logic.
Perform a road test with live data. Monitor temperature, current draw, torque, and inverter voltage.

Pro Tip: Use Mode $06 data to inspect torque deviation counters and inverter transistor test results. These hidden diagnostics often catch failures before components completely fail.

Possible Fixes & Repair Costs

Replace inverter coolant pump – Restores proper cooling; €250–€500.
Replace Drive Motor B / MG2 – Usually part of the transaxle; €1,500–€4,000+.
Replace inverter/converter assembly – Fixes voltage and waveform irregularities; €1,000–€3,000.
Replace resolver sensor – Corrects position feedback errors; €300–€700.
Repair HV connectors or wiring – Eliminates high resistance and thermal damage; €150–€400.
Update hybrid ECU software – Often inexpensive or free; typically €0–€150.
Service hybrid transaxle fluid – Helps prevent mechanical drag; €120–€180.

Always confirm the root cause before replacing expensive high-voltage components.

Can I Still Drive With P0A91?

Short-distance driving is usually possible, but not ideal. Reduced power, overheating, or warning messages may indicate that the high-voltage system is under stress. Continuing to drive with P0A91 can cause inverter overheating or sudden loss of propulsion. Have the system diagnosed as soon as possible.

Last updated: January 22, 2026

Key Takeaways

P0A91 indicates Drive Motor B isn’t producing the expected torque.
Main causes include inverter faults, motor drag, resolver issues, or wiring resistance.
Repairs range from coolant pump replacement to full transaxle replacement.
Driving with this code can risk overheating and expensive system damage.

FAQ

How do I fix P0A91?

You fix P0A91 by identifying the exact cause using hybrid-capable diagnostics. The issue may be with the motor, inverter, resolver, or coolant system. Once the faulty component is found, the repair is usually straightforward but may involve expensive high-voltage parts.

Can overheating cause P0A91?

Yes. Poor inverter cooling or a weak coolant pump can cause thermal stress that reduces motor efficiency. The ECU detects the torque deviation and sets P0A91. Restoring proper cooling often prevents the code from returning.

Is it safe to keep driving with P0A91?

Only for short distances. If you notice reduced power or hybrid warning messages, the vehicle may enter limp mode or shut down to protect itself. Continued driving risks damaging the inverter or motor.

Does P0A91 mean my hybrid battery is bad?

Not usually. This code refers to Drive Motor B performance, not the battery. While abnormal battery current can influence torque output, battery failure is not a common cause of P0A91.

Can low transaxle fluid cause P0A91?

Yes. Old or contaminated transaxle fluid increases mechanical drag on MG2, which can cause torque deviation and trigger the code. Fresh fluid helps but won’t repair internal mechanical damage.

P0A91 – Drive Motor “B” Performance