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P0738 TCM Engine Speed Output Circuit Low

The OBD2 code P0738 indicates that the Transmission Control Module (TCM) has detected a low voltage or signal issue in the engine speed output circuit, which may affect transmission performance

Definition

The OBD2 code P0738 indicates that the Transmission Control Module (TCM) has detected a low voltage or signal issue in the engine speed output circuit, which may affect transmission performance

Common causes

  • Faulty Transmission Control Module (TCM)
  • Damaged or corroded wiring or connectors in the engine speed output circuit
  • Faulty engine speed sensor
  • Low transmission fluid level
  • Transmission fluid contamination
  • Internal transmission failure
  • Poor ground connection for the TCM or sensors

Common misdiagnoses

  • Transmission fluid issues
  • Faulty transmission control module (TCM)
  • Wiring or connector problems
  • Faulty engine speed sensor
  • Incorrectly diagnosed transmission failure
  • Issues with the vehicle's computer system
  • Misinterpretation of other related codes

Troubleshooting steps

  1. 1. Check for Additional Codes

    Use an OBD-II scanner to check for any other trouble codes that may be present. This can provide more context and help identify related issues

  2. 2. Inspect Wiring and Connectors

  3. 3. Visually inspect the wiring and connectors associated with the TCM and the engine speed sensor. Look for signs of damage, corrosion, or loose connections

  4. 4. Pay special attention to the wiring harness for any fraying or breaks

  5. 5. Test the Engine Speed Sensor

  6. 6. The engine speed sensor (or crankshaft position sensor) may be faulty. Use a multimeter to test the sensor's resistance and output voltage according to the manufacturer's specifications

  7. 7. If the sensor is not functioning correctly, replace it

  8. 8. Check the TCM

  9. 9. Ensure that the TCM is receiving the correct input signals from the engine speed sensor. You may need to use a scan tool to monitor live data and check the readings from the engine speed sensor

  10. 10. If the TCM is not receiving the correct signals, it may be faulty and require replacement

  11. 11. Inspect the Transmission Fluid

  12. 12. Check the transmission fluid level and condition. Low or dirty fluid can cause shifting issues and may lead to erroneous codes

  13. 13. If the fluid is low, top it off with the correct type of fluid. If it is dirty or burnt, consider a transmission fluid change

  14. 14. Check for Ground Issues

  15. 15. Ensure that the TCM and engine speed sensor have a good ground connection. Poor grounding can lead to erratic sensor readings and codes

  16. 16. Perform a System Reset

  17. 17. After addressing any issues found, clear the trouble codes using the OBD-II scanner and take the vehicle for a test drive to see if the code returns

  18. 18. Consult Technical Service Bulletins (TSBs)

  19. 19. Check for any TSBs related to the P0738 code for your specific vehicle make and model. Manufacturers may have known issues or recommended fixes

  20. 20. Professional Diagnosis

  21. 21. If the issue persists after performing the above steps, it may be necessary to consult a professional mechanic or technician who can perform more advanced diagnostics, including checking the TCM programming and performing a thorough inspection of the transmission system. By following these troubleshooting steps, you can identify and potentially resolve the issue associated with the P0738 code