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P0921 OBD-II Trouble Code: Gear Shift Forward Actuator Circuit Range/Performance

diagnostic check engine light

P0921 code definition

When the Powertrain Control Module (PCM) detects a problem with the gear shift forward actuator circuit, a P0921 trouble code is stored. Other related trouble codes include a P0920, P0922, P0923, P0924, P0925, P0926, and P0927 trouble code.

What the P0921 code means

The P0921 trouble code is stored when the PCM detects a variation in the voltage coming from the gear shift forward actuator circuit that is outside the normal limits set by the vehicle manufacturer. In addition, the Check Engine light illuminates with the storage of this code.

What causes the P0921 code?

Some potential causes of a P0921 trouble code include a faulty gear shift forward actuator, shorted or defective wiring and connectors, a defective gear guide rail, and a defective shift shaft. A mechanical failure within the transmission can also cause storage of this code. In rare cases, a faulty PCM or Transmission Control Module (TCM) can also cause this code.

What are the symptoms of the P0921 code?

Some of the symptoms that accompany a P0921 trouble code include difficulty or the inability to shift into a forward-moving gear. Another symptom of a P0921 trouble code is a reduction in fuel efficiency.

How does a mechanic diagnose the P0921 code?

Diagnosis of a P0921 trouble code by a mechanic is accomplished through the use of an advanced scanner and a digital volt/ohm meter. The mechanic should keep in mind to address the trouble codes in the order they were stored. The easiest way to accomplish this is to download any freeze frame data and any stored trouble codes. When diagnosing a P0921 trouble code, a mechanic should perform the following:

  • Inspect any wiring, connectors, and components looking for any damage.
  • Clear the P0921 trouble code and test the system to see if the code returns.
  • If the code returns, use the digital volt/ohm meter to check the voltage and ground signal at the gear shift actuator switch.
  • If no voltage or ground signal is detected, the mechanic should disconnect the PCM and any related modules and test the continuity between the gear shift actuator switch and the battery ground.
  • In addition, the mechanic should inspect the shift shaft and the forward guide rail for damage and to make sure they are both operating properly.
  • The mechanic should then clear the P0921 trouble code again to see if it returns.
  • If the trouble code returns, the mechanic should next check the TCM for fault using the digital volt/ohm meter to check for continuity.
  • If the TCM checks out as okay, the mechanic should then inspect and check the continuity of the PCM to see if it has gone bad.
  • Finally, clear the P0921 trouble code and retest the system to see if it returns.

Common mistakes when diagnosing the P0921 code

A mistake often made by mechanics when diagnosing a P0921 trouble code is to not clear codes in the order they were stored. In addition, mechanics might also replace components that are not at fault instead of faulty wiring or connectors, resulting in a failure to fix the problem.

How serious is the P0921 code?

While a vehicle that has a P0921 trouble code stored on its PCM might still be drivable, more than likely continued use of the vehicle without getting the code fixed can result in drivability issues over time. It is best to get the P0921 trouble code fixed to prevent failure of the vehicle to shift into a forward-moving gear.

What repairs can fix the P0921 code?

To fix and clear a P0921 trouble code, the mechanic needs to perform the following repairs:

Need help with a P0921 code?

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The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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