Allison WTEC II Fault Code : 21 PID: SPN: 12 FMI: Throttle position sensor, failed low

Posted by PerDiezel Solutions on January 11, 2026

Main code 21 indicates the throttle position sensor has been retracted or extended by its linkage into an error zone. Code 21 12 is set when the ECU receives TPS counts of 14 or less. Whenever a Code 21 XX condition is detected, the system
defaults to 100 percent throttle and part throttle shifts will be abrupt.

Possible causes:
Sensor
Wiring to the sensor or the ECM
PWM signal problem

Main code 21 indicates the throttle position sensor has been retracted or extended by its linkage into an error zone.
This may be due to a fault with the sensor, or a fault in the wiring to the sensor or to the ECU. This code may also
indicate a PWM signal problem. Code 21 12 is set when the ECU receives TPS counts of 14 or less. 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.

Allison WTEC II Fault Code: 14 PID: SPN: 23 FMI: Oil level sensor, failed high (not used)

Allison WTEC II Fault Code: 14 PID: SPN: 12 FMI: Oil level sensor, failed low

Allison WTEC II Fault Code: 13 PID: SPN: 23 FMI: ECU input voltage, high

Allison WTEC II Fault Code: 13 PID: SPN: 13 FMI: ECU input voltage, medium low

Allison WTEC II Fault Code: 13 PID: SPN: 12 FMI: ECU input voltage, low

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WARNING!