WOMS
P code powertrain Medium severity

P0594 Thermostat Heater Control Circuit Intermittent

The OBD2 code P0594 indicates an intermittent issue with the thermostat heater control circuit, which may affect the engine's ability to reach and maintain the optimal operating temperature

Definition

The OBD2 code P0594 indicates an intermittent issue with the thermostat heater control circuit, which may affect the engine's ability to reach and maintain the optimal operating temperature

Common causes

  • Faulty thermostat
  • Damaged wiring or connectors
  • Short circuit in the heater control circuit
  • Failed engine control module (ECM)
  • Poor ground connection
  • Corroded terminals
  • Intermittent electrical connection

Common misdiagnoses

  • Faulty thermostat
  • Wiring issues
  • Engine coolant temperature sensor failure
  • Powertrain control module (PCM) malfunction
  • Cooling system leaks
  • Faulty heater control valve
  • Incorrect coolant level or type
  • Electrical connector problems

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 the Thermostat

  3. 3. Check the thermostat itself for proper operation. If it is stuck open or closed, it may need to be replaced

  4. 4. Ensure that the thermostat is the correct type for your vehicle

  5. 5. Examine the Wiring and Connectors

  6. 6. Inspect the wiring harness and connectors associated with the thermostat heater control circuit for any signs of damage, corrosion, or loose connections

  7. 7. Look for frayed wires, broken connectors, or any signs of wear that could cause intermittent connectivity

  8. 8. Test the Thermostat Heater

  9. 9. Use a multimeter to check the resistance of the thermostat heater. Compare the readings to the specifications provided in the vehicle's service manual

  10. 10. If the resistance is out of range, the thermostat may need to be replaced

  11. 11. Check the Engine Coolant Temperature (ECT) Sensor

  12. 12. Inspect the ECT sensor for proper operation. A faulty ECT sensor can cause incorrect readings and affect the thermostat's operation

  13. 13. Test the ECT sensor with a multimeter to ensure it is functioning correctly

  14. 14. Inspect the Engine Control Module (ECM)

  15. 15. Check for any software updates or recalls related to the ECM that may address the issue

  16. 16. If all other components check out, the ECM itself may be faulty and require further diagnosis or replacement

  17. 17. Clear Codes and Test Drive

  18. 18. After performing any repairs or replacements, clear the trouble codes using the OBD-II scanner

  19. 19. Take the vehicle for a test drive to see if the code reappears. Monitor the engine temperature and performance during the drive

  20. 20. Consult a Professional

    If the issue persists after performing these steps, it may be best to consult a professional mechanic for further diagnosis. They may have specialized tools and knowledge to identify more complex issues. By following these troubleshooting steps, you should be able to identify and resolve the underlying cause of the P0594 code