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Diagnosis & Repair Procedure

NOTE: For better performance and results of this diagnostic procedure, usage of appropriate wiring diagram is recommended. See ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS article in ELECTRICAL.
NOTE: Perform all voltage and resistance tests using a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure. Use an ohmmeter to isolate wiring harness shorts or opens.
  1. If engine system check has been performed, go to next step. If engine system check has not been performed, perform TEST D: ENGINE SYSTEM CHECK  under SYSTEM TESTS then go to next step.
  2. Turn ignition switch to ON position. Using scan tool, observe and record ECT sensor value (engine cold). Start engine and allow it to reach normal operating temperature, observe and record ECT sensor values (engine warm). If ECT sensor values does not vary from 1°F-167°F (1°C-75°C), go to next step. If ECT sensor values vary from 1°F-167°F (1°C-75°C), problem is intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES article. After repair, repeat DTC CONFIRMATION TEST .
  3. Turn ignition switch to LOCK position. Disconnect ECT sensor harness connector. See Figure. Disconnect ECM harness connectors. See Figure. Turn ignition switch to ON position. Measure voltage between ECM harness connector C46 terminal No. 12 and ECT sensor harness connector C21 terminal No. 2 (Light Green wire). See Figure and Figure . If voltage is zero volt, go to next step. If any voltage is found, check Light Green wire for short to other circuits. If short to other circuits is found, repair as necessary and repeat DTC CONFIRMATION TEST . If no short to other circuits is found, substitute ECM with a known good unit and repeat DTC CONFIRMATION TEST .
  4. Turn ignition switch to LOCK position. Measure resistance between ECM harness connector C46 terminal No. 12 and ECT sensor harness connector C21 terminal No. 2 (Light Green wire). See Figure and Figure . If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair short circuit in Light Green wire between ECM and ECT sensor. After repair, repeat DTC CONFIRMATION TEST .
  5. Reconnect ECM harness connector and ensure proper terminal connection. Measure resistance between ground and ECT sensor harness connector C21 terminal No. 1 (Orange wire). See Figure. If resistance is 10 ohms or more, go to next step. If resistance is less than 10 ohms, go to step  7.
  6. Backprobe and measure resistance between ground and ECM harness connector C46 terminals No. 27 (Orange wire). See Figure. If resistance is less than 5 ohms, repair open or short circuit in Orange wire between ECM and ECT sensor. After repair, repeat DTC CONFIRMATION TEST . If resistance is 5 ohms or more, check ECM ground circuits (Black/Orange wire) and (Black wire) for open or short between ECM and ground. If ground circuits are faulty, repair as necessary and repeat DTC CONFIRMATION TEST . If ground circuits are okay, substitute ECM with a known good unit and repeat DTC CONFIRMATION TEST .
  7. Perform ECT sensor component test. See ENGINE COOLANT TEMPERATURE SENSOR under ENGINE SENSORS & SWITCHES in SYSTEM & COMPONENT TESTING article. If ECT sensor is faulty, replace ECT sensor. See ENGINE COOLANT TEMPERATURE SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL & INSTALLATION article. After repair, repeat DTC CONFIRMATION TEST . If ECT sensor is okay, substitute ECM with a known good unit and repeat DTC CONFIRMATION TEST .
  8. Check thermostat operation. If heater takes a long time to become effective, or coolant flows into radiator on cold start, check thermostat operation. If thermostat operates properly, go to next step. If thermostat is faulty, repair or replace as necessary and repeat DTC CONFIRMATION TEST .
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