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DTC 123: Engine Coolant Temperature Sensor Signal

NOTE: For connector terminal and relay location identification, see Figure-Figure and WIRING DIAGRAMS article. For component locations, see COMPONENT LOCATIONS .
NOTE: Before performing test, disconnect engine cooling fan relay connector "A". DTC 514 (engine cooling fan half speed) signal will be set. After testing, reattach connector.
  1. If ECM receives a signal from ECT sensor which indicates a temperature less than -49°F (-45°C), or greater than 300°F (150°C), this is interpreted as a fault and DTC 123 is set.
  2. If fault is intermittent and signal is too high, check all connectors for an intermittent open or short circuit to voltage. If fault is intermittent and signal is too low, check wiring between ECT sensor connector terminal No. 1 and ECM connector terminal A31 for an intermittent short to ground. If fault is intermittent and signal is faulty, check ECM and ECT sensors for good terminal contact. Repair as necessary. If fault is permanent and signal is faulty, go to next step. If fault is permanent and signal is low or high, go to step 7).
  3. Start and run engine until it reaches normal operating temperature. If engine fails to reach normal operating temperature, check cooling system. Repair as necessary. If engine warms up to normal operating temperature, go to next step.
  4. Turn ignition on. Disconnect ECT sensor connector. Connect voltmeter between ECT sensor connector terminals. If voltmeter reads about 5 volts, go to step 6). If reading is not about 5 volts, go to next step.
  5. Turn ignition off. Disconnect ECT sensor connector. Check ECM connector for good terminal contact. Repair as necessary. Reconnect ECM. Turn ignition on. Using scan tool, go into scrolling value list. Read ECT sensor voltage. If reading is about 5 volts, problem was poor ECM connector contact. If reading is not about 5 volts, check wiring between ECT sensor terminal No. 1 and ECM connector terminal A31 for a short to voltage.
  6. Turn ignition off. Wait 150 seconds. Disconnect ECT sensor connector. Connect ohmmeter between ECT sensor connector terminal No. 2 and ground. If ohmmeter reads about zero ohms, install new ECT sensor. If ohmmeter does not read about zero ohms, check ECM connector for good terminal contact. Repair as necessary. If ohmmeter now reads about zero ohms, problem was poor ECM connector terminal contact. If ohmmeter still does not read about zero ohms, check wiring between ECT sensor terminal No. 2 and ECM connector terminal A18 for an open circuit.
  7. If fault is permanent and signal is too low, go to step 13). If fault is permanent and signal is too high, turn ignition off. Wait 150 seconds, then disconnect ECT sensor. Connect an ohmmeter between ECT sensor connector No. 2 and ground.
    NOTE: Fan run-on must be allowed to finish before taking resistance measurements.
  8. If ohmmeter shows about zero ohms, go to next step. If ohmmeter does not show about zero ohms, go to step 10).
  9. Turn ignition on. Disconnect ECT sensor connector. Connect voltmeter between ECT sensor connector terminal No. 1 and 2. If voltmeter shows about 5 volts, go to step 11). If voltmeter shows less than 5 volts, go to step 12). If voltmeter shows more than about 5 volts, check wiring between ECT sensor connector terminal No. 1 and ECM connector terminal A31 for a short to voltage, then check resistance between ECT sensor terminals. Ohmmeter should show about 2200 ohms. If resistance is incorrect, retest using a known-good ECT sensor.
  10. Ensure ignition is off. Disconnect ECT sensor connector. Check ECM connector for poor contact and/or oxidation. Reconnect ECM connector. Connect an ohmmeter between ECT sensor connector terminal No. 2 and ground. If ohmmeter shows about zero ohms, DTC is caused by poor contact in ECM connector. If ohmmeter does not show about zero ohms, check wiring between ECT sensor connector terminal No. 2 and ECM connector terminal A18 for an open circuit.
  11. Ensure ignition is off. Disconnect ECT sensor connector. Connect an ohmmeter between ECT sensor connector terminals No. 1 and 2. If ohmmeter does not show about 2200 ohms, repeat test using a known-good ECT sensor. If ohmmeter does show about 2200 ohms, check for poor contact at ECT sensor connector.
  12. Ensure ignition is off. Disconnect ECT sensor connector. Ensure ECM connector has good terminal contact. Reconnect ECM connector. Turn ignition on. Connect voltmeter between ECT sensor connector terminals No. 1 and 2. If voltmeter shows about 5 volts, check for poor contact at ECM connector. If voltmeter does not show about 5 volts, check wiring between ECT sensor connector terminal No. 1 and ECM connector terminal A31 for an open circuit.
  13. If fault is permanent and signal is too low, turn ignition off. Disconnect ECT sensor. Turn ignition on. Using scan tool, go into scrolling values list. Read voltage between ECT sensor. If scan tool shows about 5 volts, retest system using a known-good ECT sensor. If voltmeter does not show about 5 volts, check wiring between ECT sensor terminal No. 1 and ECM connector terminal A31 for a short to ground.
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