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DTC 123: Engine Coolant Temperature Sensor Signal

  1. Check for additional DTCs. If DTCs 314 and 411 are present, go to next step. If DTCs 314 and 411 are not present, go to step 4).
  2. Check common Engine Coolant Temperature (ECT) sensor, Camshaft Position (CMP) sensor, and Throttle Position (TP) sensor terminal A18 on ECM for poor terminal contact. Repair as necessary, then go to next step.
  3. Ensure ignition is off. Ensure all connectors are connected and components are installed. Connect Multimeter (999 6525) with Temperature Box (999 5281) and Temperature Sensor (999 5282). Carefully remove expansion tank cover. Using a hose clip, clamp hose between radiator and thermostat housing. Disconnect hose from thermostat housing. Insert sensor in thermostat housing and reinstall hose. Remove hose clip. Start and run engine so it reaches normal operating temperature and thermostat opens. Using scan tool, go into scrolling values. Read off engine coolant temperature. If engine coolant temperature is within normal range, system is okay. If engine coolant temperature is not within normal range, check engine cooling system.
  4. If status message reads PERMANENT FAULT, SIGNAL TOO HIGH, go to next step. If status message reads PERMANENT FAULT, SIGNAL TOO LOW, go to step 11). If status message reads PERMANENT FAULT, FAULTY SIGNAL, go to step 12). If status message reads INTERMITTENT FAULT, SIGNAL TOO HIGH, go to step 18). If status message reads INTERMITTENT FAULT, SIGNAL TOO LOW, check wiring between ECT sensor terminal No. 1 and ECM terminal A31. Repair as necessary.
    NOTE: Before testing, disconnect engine cooling fan relay connector "A". DTC 514 (engine cooling fan half speed) signal will be set. After testing, reattach connector.
  5. Turn ignition off. Wait about 2 minutes. Disconnect ECT sensor. Connect ohmmeter between ECT sensor connector terminal No. 2 (ECM side) and ground. Ensure cooling fan run-on is finished before taking resistance measurements. If ohmmeter shows about zero ohms, go to next step. If ohmmeter does not show about zero ohms, go to step 7).
  6. Turn ignition on. Disconnect ECT sensor. Connect voltmeter between ECT sensor terminals No. 1 and 2. If voltmeter reads about 5 volts, go to step 8). If voltmeter reads less than 5 volts, go to step 10). If voltmeter reads greater than 5 volts, check wiring between ECT sensor terminal No. 1 and ECM terminal A31 for a short circuit to voltage, then go to step 9).
  7. Turn ignition off. Disconnect ECT sensor. Disconnect ECM connector and check for poor terminal contact. Repair as necessary. Connect ohmmeter between ECT sensor connector terminal No. 2 (ECM side) and ground. If ohmmeter reads about zero ohms, DTC was caused by poor terminal contact at ECM. If ohmmeter does not read about zero ohms, check wiring between ECT sensor connector terminal No. 2 and ECM terminal A18 for an open circuit.
  8. Turn ignition off. Disconnect ECT sensor. Connect ohmmeter between ECT sensor connector terminals No. 1 and 2 (sensor side). Ohmmeter should read about 2.2 k/ohms at 77°F (25°C). If reading is to specification, DTC is caused by poor terminal contact at ECT sensor connector. If reading is not to specification, replace ECT sensor.
  9. Turn ignition off. Disconnect ECT sensor. Connect ohmmeter between ECT sensor connector terminals No. 1 and 2 (ECT sensor side). If ohmmeter reads about 2.2 k/ohms at 77°F (25°C), ECT sensor is okay. If ohmmeter does not read about 2.2 k/ohms at 77°F (25°C), replace ECT sensor.
  10. Turn ignition off. Disconnect ECT sensor. Disconnect ECM connector and check for poor terminal contact. Repair as necessary. Reconnect ECM connector. Turn ignition on. Connect voltmeter between ECT sensor connector terminals No. 1 and 2 (ECM side). If voltmeter reads about 5 volts, DTC was caused by poor terminal contact at ECM connector. If voltmeter does not read about 5 volts, check wiring between ECT sensor connector terminal No. 1 and ECM terminal A31 for an open circuit. Repair as necessary.
  11. Turn ignition on. Using scan tool, go into scrolling values. Disconnect ECT sensor. Using scan tool, read off voltage across ECT sensor. If scan tool reads about 5 volts, replace ECT sensor. If scan tool does not read about 5 volts, check wiring between ECT sensor connector terminal No. 1 and ECM terminal A31 for a short to ground.
  12. Ensure ignition is off. Connect Multimeter (999 6525 ) with Temperature Box (999 5281 ) and Temperature Sensor (999 5282 ). Carefully remove expansion tank cover. Using a hose clip, clamp hose between radiator and thermostat housing. Disconnect hose from thermostat housing. Insert sensor in thermostat housing and reinstall hose. Remove hose clip. Using scan tool, go into scrolling values. Start and run engine so it reaches normal operating temperature and thermostat opens. Read off engine coolant temperature. If engine coolant temperature is within normal range, go to next step. If engine coolant temperature is not within normal range, go to step 14).
  13. This DTC can be stored if thermostat is defective or if tolerance of ECT sensor is incorrect within measured range. Using scan tool, read off frozen values. If frozen value for ECT sensor is outside its range, then ECT sensor is defective. Ensure thermostat opens at correct temperature. Replace both ECT sensor and thermostat if no faults are detected in either component.
  14. Turn ignition on. Using scan tool, go into scrolling values list. Read off voltage across ECT sensor. If reading is about 5 volts, go to step 16). If reading is not about 5 volts, go to next step.
  15. Turn ignition off. Disconnect ECT sensor. Disconnect ECM connector and check for poor terminal contact. Repair as necessary. Reconnect ECM connector. Turn ignition on. Using scan tool, go into scrolling values list. Read off voltage across ECT sensor. If about 5 volts are present, DTC was caused by poor terminal contact at ECM connector. If about 5 volts are not present, check wiring between ECT sensor terminal No. 1 and ECM terminal A31 for a short circuit.
  16. Turn ignition off. Wait about 2 minutes. Disconnect ECT sensor. Connect ohmmeter between ECT sensor terminal No. 2 (ECM side) and ground. If ohmmeter reads zero ohms, replace ECT sensor. If ohmmeter does not read zero ohms, go to next step.
  17. Turn ignition off. Disconnect ECT sensor. Disconnect ECM connector and check for poor terminal contact. Repair as necessary. Reconnect ECM connector. Connect ohmmeter between ECT sensor connector terminal No. 2 (ECM side) and ground. If ohmmeter reads about zero ohms, DTC was caused by poor terminal contact at ECM connector. If ohmmeter does not read about zero ohms, check wiring between ECT sensor terminal No. 2 and ECM connector terminal A18 for an open circuit.
  18. Check ECT and ECM connectors for poor terminal contact. Repair as necessary. Check wiring between ECT sensor connector terminal No. 1 and ECM terminal A31 for an intermittent open circuit. Check wiring between ECT sensor connector terminal No. 2 and ECM terminal A18 for an intermittent open circuit. Repair as necessary.
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