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Code 21: Engine Coolant Temperature (ECT) Sensor

NOTE: For component terminal identification, see TERMINAL IDENTIFICATION  . For wiring diagrams, see WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section.
  1. If using scan tester, go to step 2). Remove Engine Coolant Temperature (ECT) sensor from intake manifold. Submerge temperature sensing portion of ECT sensor in hot water. Using DVOM, check resistance across ECT sensor terminals. See ECT SENSOR RESISTANCE SPECIFICATIONS  table. If resistance is not as specified, replace ECT sensor. If resistance is as specified, go to step 3).
    ECT SENSOR RESISTANCE SPECIFICATIONS

    Water Temperature Approximate Ohms
    32°F (0°C) 5800
    68°F (20°C) 2400
    104°F (40°C) 1100
    176°F (80°C) 300
  2. Turn ignition switch to ON or RUN position. Using a thermometer, check engine compartment ambient temperature. Using scan tester, read Engine Coolant Temperature (ECT) sensor voltage. See ECT SENSOR VOLTAGE SPECIFICATIONS  table. If voltage is not within specifications, replace ECT sensor. If voltage is within specification, go to next step.
    ECT SENSOR VOLTAGE SPECIFICATIONS

    Ambient Temperature Standard Value: °F (°C)
    -4°F (-20°C) -20°C
    32°F (0°C) 0°C
    68°F (20°C) 20°C
    104°F (40°C 40°C
    176°F (80°C) 80°C
  3. Disconnect ECT sensor connector. Using DVOM, check continuity between chassis ground and specified connector terminal. See ECT SENSOR GROUND CIRCUIT TERMINAL IDENTIFICATION  table. If continuity does not exist, repair wiring harness as necessary. If continuity exists, go to next step.
    ECT SENSOR GROUND CIRCUIT TERMINAL IDENTIFICATION

    Application Terminal No.
    1.5L 1
    1.8L 2
  4. Disconnect ECT sensor connector and ECM connector. Check continuity between specified ECT sensor connector terminals and ECM connector terminals. See ECT SENSOR-TO-ECM WIRING HARNESS TERMINAL IDENTIFICATION  table. If continuity does not exist, repair wiring harness as necessary. If continuity exists, go to next step.
    ECT SENSOR-TO-ECM WIRING HARNESS TERMINAL IDENTIFICATION

    Application ECT Sensor Terminal No. ECM Terminal No.
    1.5L 2 63
    1.8L 1 63
  5. Turn ignition switch to ON position. Check voltage between chassis ground and specified ECT sensor connector terminal. See ECT SENSOR VOLTAGE CIRCUIT IDENTIFICATION  table. If voltage is not 4.5-4.9 volts, replace ECM. If voltage is as specified, condition required to set trouble code is not present at this time. Test is complete. Intermittent problem may exist. See TESTS W/O CODES article in the ENGINE PERFORMANCE Section.
    ECT SENSOR VOLTAGE CIRCUIT IDENTIFICATION

    Application Terminal No.
    1.5L 1
    1.8L 2
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