Error Code P0904 Is defined as Transmission Gate Select Position Circuit Malfunction. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to the present. This code appears in vehicles with transmission fluid temperature sensor. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

This error code means that the Electronic Control Module (ECM) has determined a problem with the gate select position sensor circuit.

Other related codes that may accompany this code include:

  • Error Code P0905
  • Error Code P0906
  • Error Code P0907
  • Error Code P0908

Today, most vehicles come with various sensors in the ECM and Transmission Control Module (TCM) that control the transmission performance.  One of these sensors is called the Gate Select Position sensor, which tells the vehicle’s ECM and TCM gear position. These sensors also work by monitoring the brake slippage and detect clutch application in a manual vehicle. If the ECM doesn’t get the right signals from the sensor, it will register this error code.

Common Symptoms

  • Cruise control not working
  • Check Engine light activated

Possible Causes

In most cases, bad electrical connections in the circuit are the common cause of this code, such as loose connections or corroded or damaged wiring harness. Other common causes include:

  • Misaligned sensor
  • Faulty sensor

How to Check

Start the diagnosing by checking the alignment of the sensor. A misaligned sensor is a common issue after transmission repair. Thus, sensors may need to be rotated to a neutral baseline for the gate select position to be detected properly.

After performing the repair, make sure to clear the codes and retest the system.

If the code reemerges, then check all related electrical components. Look for signs of corrosion, damage, or loose wirings or connectors—repair as needed. Then clear the codes and retest again.

If neither works, the sensor is probably the problem.

How to Fix

  • Realignment of sensors that were in the wrong position
  • Repair or replacement of damaged electrical components
  • Replacement of the faulty sensor

This code can make the vehicle unsafe for driving. Thus it is important to address this code as soon as it is detected.

When diagnosing this code, the most common mistake is forgetting to disconnect the PCM and the cruise control module when testing the circuit resistance.

After each repair, make sure to clear the codes, retest, and take the vehicle for a test drive to see if the code comes back.