In order to put your vehicle into gear, the computer needs a series of sensors and electric motors to make it happen. The gear shift forward actuator combines all of these things into a single assembly that can be controlled by the ECU/TCM. If it is not functioning properly, a P0921 DTC will be saved.
How Serious is the Code?
The P0921 diagnostic trouble code is actually pretty serious, because it needs the TCM cannot engage a forward gear. Obviously, this will cause the transmission to behave erratically and possibly will not go into gear when you tell it to.
Symptoms of a P0921 include:
- Transmission behaves erratically
- Forward gears cannot be engaged or disengaged
- Wiring and/or connector damage
- Faulty gearshift forward actuator
- Damage to the gear guide rail
- Damage to the shift shaft
- Internal mechanical problems
- ECU/TCM issues or failure
How to Diagnose the Code?
In order to diagnose a P0921 OBDII trouble code, a mechanic will have to inspect the wiring and connectors for signs of damage and corrosion. The gear shift forward actuator will also need to be checked, along with the condition of the gear guide rail and shift shaft. If no problems have been found up to this point, there may be internal mechanical problems that will need to be addressed.
What Repairs Will Fix P0921?
- Repair or replace any damage wiring or connectors
- Replace gearshift forward actuator
- Repair or replace gear guide rail
- Repair or replace shift shaft
- Repair internal mechanical fault(s)
- Reprogram or replace ECU/TCM
- P0920 – Gear Shift Forward Actuator Circuit/Open
- P0922 – Gear Shift Forward Actuator Circuit Low
- P0923 – Gear Shift Forward Actuator Circuit High
- P0924 – Gear Shift Reverse Actuator Circuit/Open
- P0925 – Gear Shift Reverse Actuator Circuit Range/Performance
- P0926 – Gear Shift Reverse Actuator Circuit Low
- P0927 – Gear Shift Reverse Actuator Circuit High