If your unit has a AC or DC controlled motor there is a temp sensor inside the motor that can go bad. Note: there is also a temp sensor inside the motor controller. Now the following troubleshooting information only suggests to test the motor temp and replace the sensor OR the problem may be in the dash display. I would check the motor sensor and wiring harness's and connectors before i went to the dash display replacement.

-Fault code 91-
Probable Causes and Test Procedures
* Use dash display diagnostics to verify motor overtemperature
switch is activated.
* Reduce truck operation until motor cools.
* If fault is signaled with cold motor, use dash display
diagnostic to verify motor temperature.
Replace motor temperature sensor.
Replace dash display.