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Code 21 - Coolant Temperature Sensor

NOTE: Refer to illustration for ECM connector terminal identification. See Figure .
  1. Remove collector cover and air intake boot. Disconnect coolant temperature sensor connector. Using an ohmmeter, check sensor resistance. Resistance should be 30,000 ohms at 68°F (20°C) and 400 ohms at 176°F (80°C). If resistance is as specified, go to next step. If resistance is not as specified, replace coolant temperature sensor.
  2. Turn ignition off. Disconnect ECM connectors. Check resistance between ECM connector terminal A3 and sensor connector terminal No. 1 (Gray/Yellow wire), and between ECM connector terminal A21 and sensor connector terminal No. 2 (Blue/Black wire)
  3. Resistance should be zero ohms for all circuits. If resistance is not as specified, repair appropriate circuit as necessary. If resistance is as specified, check ECM connector terminals for good contact. Repair as necessary. If connector terminals are okay, replace ECM.
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