What Does P0A91 Mean?
DTC P0A91 indicates drive motor 'B' (typically the secondary motor/generator in a hybrid system, or the rear motor in an AWD EV) is not performing within expected parameters. The hybrid control module has detected that motor B's output deviates from commanded values. In many hybrid systems, motor B serves dual roles — generating electricity during braking and providing additional traction. Loss of motor B performance can affect regenerative braking and all-wheel-drive capability.
Safety Warning
Loss of motor B performance can reduce or eliminate regenerative braking capability, increasing reliance on friction brakes and potentially increasing stopping distances. In AWD configurations, loss of the secondary motor affects traction in slippery conditions. The vehicle may enter a reduced-power mode.
Common Causes
25%
Degraded motor stator windings with shorted turns or insulation breakdown
25%
Faulty resolver/position sensor for motor B providing inaccurate feedback
20%
Inverter B malfunction causing incorrect phase current control
15%
Mechanical bearing wear or damage in the motor B assembly
15%
High-resistance connection at motor B phase terminals or inverter output
Diagnostic Steps
1
Review freeze frame data to determine operating conditions when the fault occurred — compare motor B commanded vs. actual torque and speed.
2
Test the motor B resolver/position sensor signals for accuracy, noise, and consistency using the scan tool or oscilloscope.
3
Measure motor B phase-to-phase resistance and insulation resistance — all three phases should be balanced within manufacturer tolerances.
4
Inspect high-voltage connections at motor B and its dedicated inverter for signs of arcing, overheating, or corrosion.
5
Check the motor and inverter coolant temperature — verify the cooling system is maintaining proper temperatures for motor B.
6
If available, run a manufacturer-specific motor performance test to isolate whether the motor, inverter, or sensor is the root cause.
Estimated Repair Cost
$500 - $5,000
Parts + labor, varies by vehicle and location
DTC P0A91 indicates drive motor 'B' (typically the secondary motor/generator in a hybrid system, or the rear motor in an AWD EV) is not performing within expected parameters. The hybrid control module has detected that motor B's output deviates from commanded values. In many hybrid systems, motor B ...
The most common cause of P0A91 (Drive Motor "B" Performance) is: Degraded motor stator windings with shorted turns or insulation breakdown
Typical repair costs for P0A91 range from $500 to $5,000, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Loss of motor B performance can reduce or eliminate regenerative braking capability, increasing reliance on friction brakes and potentially increasing stopping distances. In AWD configurations, loss of the secondary motor affects traction in slippery conditions. The vehicle may enter a reduced-power mode.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0A91 to identify the root cause.
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Quick Info
Category
Powertrain
System
Hybrid/EV Drive Motor System
Difficulty
Type
Generic (SAE)
Recommended Tools
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