Definition of Code P0921
When the Powertrain Control Module (PCM) detects an issue with the forward shift actuator circuit, a fault code P0921 is recorded. Other related fault codes include P0920, P0922, P0923, P0924, P0925, P0926, and P0927.
What Does Code P0921 Mean
The fault code P0921 is recorded when the PCM detects a variation in voltage from the forward shift actuator circuit that is outside the normal limits set by the vehicle manufacturer. Additionally, the Check Engine light illuminates with the storage of this code.
What Causes Code P0921?
Some potential causes of a P0921 fault code include a faulty forward shift actuator, shorted or defective wiring and connectors, a defective speed guide rail, and a faulty shift shaft. A mechanical failure within the transmission can also lead to the storage of this code. In rare cases, a faulty PCM or Transmission Control Module (TCM) may also cause this code.
What Are the Symptoms of Code P0921?
Some of the symptoms accompanying a P0921 fault code include difficulty or inability to shift into a forward gear. Another symptom of a P0921 fault code is reduced fuel consumption.
How Does a Mechanic Diagnose Code P0921?
Diagnosing a P0921 fault code by a mechanic is accomplished using an advanced scanner and a digital voltmeter. The mechanic must keep in mind to address fault codes in the order they were stored. The easiest way to do this is to download all freeze frame data and stored fault codes. When diagnosing a P0921 fault code, a mechanic should perform the following steps:
- Inspect the wiring, connectors, and components for damage.
- Clear the P0921 fault code and test the system to see if the code returns.
- If the code returns, use the digital voltmeter/ohmmeter to check the voltage and ground signal at the shift actuator switch.
- If no voltage or ground signal is detected, the mechanic must disconnect the PCM and all associated modules and test the continuity between the shift actuator switch and the battery ground.
- Additionally, the mechanic should inspect the shift shaft and forward guide rail for damage and ensure both are functioning properly.
- The mechanic should then clear the P0921 fault code again to see if it returns.
- If the fault code returns, the mechanic must then check the TCM for any faults using the digital voltmeter/ohmmeter to verify continuity.
- If the TCM checks out fine, the mechanic should then inspect and check the continuity of the PCM to see if it has malfunctioned.
- Finally, clear the P0921 fault code and test the system again to see if it returns.
Common Mistakes When Diagnosing Code P0921
A common mistake made by mechanics when diagnosing a P0921 fault code is not clearing the codes in the order they were stored. Additionally, mechanics may replace components that are not at fault instead of defective wiring or connectors, resulting in an inability to resolve the issue.
How Severe Is Code P0921?
Although a vehicle with a stored P0921 fault code on its PCM may still be drivable, continued use of the vehicle without fixing the code can lead to driving issues over time. It is best to correct the P0921 fault code to prevent the vehicle from failing to shift into a forward gear.