The trouble code P0904 lights up when the electronic control module, or ECM, has sensed a problem with the gate select position censor circuit. This code may equally be accompanied by the P0905, P0906, P0907, or P0908 codes.
What does the P0904 code mean?
In most cars manufactured today, the TCM and ECM make use of various sensors to monitor and control the operation of the transmission. One of those sensors is the gate select position sensor, which instructs the TCM and ECM what gear position the driver has put the car in. These sensors are equally used to monitor brake slippage and to check clutch application in a manual vehicle. If the ECM is not getting the correct signal from this sensor, it will trigger the P0904 code.
What are the causes of the P0904 code?
Most times, bad electrical connections within the circuit are to blame. This may include corroded or bad wire harnesses, or loose connections. However, a misaligned sensor can equally cause this issue. As a last resort, a faulty sensor may be thought as the cause of the P0904 code.
What are the signs of the P0904 code?
Before the P0904 code is identified, the cruise control will often stop working. In most instances, the Check Engine light will activate within two to three drive cycles after the PCM has detected the problem.
How should a mechanic diagnose the P0904 code?
After an OBD-II scanner has detected the P0904 code, a technician should start by checking the alignment of the sensor. After a transmission repair, it is very common for sensors to be misaligned. They may have to be rotated to a neutral baseline in order for the gate select position to be properly identified.
After this repair is carried out, the mechanic should clear the code and retest the system. If the code resurfaces, they should then check all electrical parts, looking for any loose, corroded, damaged, or otherwise faulty wiring or connectors. These should be changed and then the system should be cleared and inspected again.
If neither of these repairs offers the correct diagnosis, then the sensor is probably culprit.
Common mistakes made when diagnosing the P0904 code
When you are diagnosing the underlying cause of the P0904 code, the most common mistake is forgetting to disconnect the PCM and the cruise control module when testing the circuit resistance. It is equally important to clear the code and do a road test after each potential solution is tried. If the most common repairs do not stop the Check Engine light from reappearing, it could be a sign of an intermittent problem that requires an in-depth check and servicing.
How serious is the P0904 trouble code?
If the P0904 code is being identified, a vehicle should be fixed immediately. Without the ability to properly detect the gear, the can can become unsafe to drive, or even get become inoperable.
What repairs can correct the P0904 code?
Fix to address the P0904 code include:
- Realigning any sensors that were not in the correct position.
- Replacing or repair any electrical parts that were not operating correctly.
- Replacing a faulty sensor.