P0904 code definition
The P0904 code indicates that the electronic control module, or ECM, has detected a problem with the gate select position censor circuit. This code may also be accompanied by the P0905, P0906, P0907, or P0908 codes.
What the P0904 code means
In most vehicles manufactured today, the TCM and ECM use various sensors to monitor and control the performance of the transmission. One of those sensors is the gate select position sensor, which tells the TCM and ECM what gear position the driver has put the vehicle in. These sensors are also used to monitor brake slippage and to detect clutch application in a manual vehicle. If the ECM is not receiving the correct signal from this sensor, it will register the P0904 code.
What causes the P0904 code?
Most often, bad electrical connections within the circuit are to blame. This may include corroded or damaged wire harnesses, or loose connections. However, a misaligned sensor can also cause this issue. As a last resort, a faulty sensor may be suspected as the cause of the P0904 code.
What are the symptoms of the P0904 code?
Before the P0904 code is detected, the cruise control will usually stop working. In most cases, the Check Engine light will activate within two to three drive cycles after the PCM has detected the issue.
How does a mechanic diagnose the P0904 code?
After an OBD-II scanner detects the P0904 code, a technician should begin by checking the alignment of the sensor. After a transmission repair, it is common for sensors to be misaligned. They may need to be rotated to a neutral baseline in order for the gate select position to be properly detected.
After this repair is performed, the mechanic should clear the code and retest the system. If the code reappears, they should then check all electrical components, looking for any loose, corroded, damaged, or otherwise faulty wiring or connectors. These should be replaced and then the system should be cleared and checked again.
If neither of these repairs gives the correct diagnosis, then the sensor is probably faulty.
Common mistakes when diagnosing the P0904 code
When 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 circuit resistance. It is also important to clear the code and do a road test after each potential solution is attempted. If the most common repairs do not stop the Check Engine light from reappearing, it can be a sign of an intermittent issue that requires an in-depth inspection and servicing.
How serious is the P0904 code?
If the P0904 code is being detected, a vehicle should be repaired immediately. Without the ability to properly detect the gear, the vehicle can become unsafe to drive, or may become inoperable.
What repairs can fix the P0904 code?
Repairs to address the P0904 code include:
- Realign any sensors that were not in the correct position.
- Replace or repair any electrical components that were not operating correctly.
- Replace a faulty sensor.
Need help with a P0904 code?
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