Allison WTEC II Fault Code: 21 PID: SPN: 23 FMI: Throttle position sensor, failed high

Main code 21 indicates the throttle position sensor has been retracted or extended by its linkage into an error zone. Code 21 23 is set when the ECU senses TPS counts of 233?255. Whenever a Code 21 XX condition is detected, the system defaults to 100 percent throttle and part throttle shifts will be abrupt.

Possible causes:
1. Sensor
2. Wiring to the sensor or the ECM
3. PWM signal problem