P0A91: Drive Motor B Performance

P0A91 is set when the second electric drive motor (motor B) of a hybrid or electric vehicle does not deliver the expected speed or torque, although no electrical circuit fault is detected. Motor B exists only on systems with two traction motors, such as power-split or dual-motor layouts. CARB requires hybrid OBD systems to monitor electric motor performance. No manufacturer documents this code publicly; GM's tables describe the motor A equivalent P0A90 as a comparison between motor speed and a reference speed, which illustrates the approach. Causes range from inverter or resolver faults to mechanical damage. The vehicle usually limits power or enters a reduced mode. P0A79 covers inverter B performance.

High severity. Can I keep driving?If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

Symptoms

Hybrid/EV system warning and MIL
Reduced power or limp mode

Most likely causes

Inverter B fault limiting motor current
Motor position sensor (resolver) signal error
Mechanical damage to motor B or its gearing

What to check, in order

Start with the simplest. You can tick off each step.

Read the codes with a hybrid-compatible scanner and write down the freeze frame data
Compare the reading with other engine data while varying the engine speed.
Make sure the battery cooling air intake is clean
Check the 12 V auxiliary battery, which can also trigger warnings
Leave any high-voltage measurements to a specialized repair shop

Good to know

It can trigger limp mode.
It affects emissions.

What each character of P0A91 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
A: Hybrid or air and fuel
91: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0A91?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code P0A91?

The most common causes are: inverter B fault limiting motor current; motor position sensor (resolver) signal error; mechanical damage to motor B or its gearing.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.