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Code 21: Coolant Temperature Sensor

NOTE: For component terminal identification, see TERMINAL IDENTIFICATION  . For wiring diagrams, see WIRING DIAGRAMS article.
  1. If using scan tester, go to step 2. Remove Coolant Temperature Sensor (CTS) from intake manifold. Submerge temperature sensing portion of CTS in hot water. Using DVOM, check resistance across CTS terminals. See CTS RESISTANCE SPECIFICATIONS  table. If resistance is not as specified, replace CTS. If resistance is as specified, go to step 3.
    CTS 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 Coolant Temperature Sensor (CTS) voltage. See CTS VOLTAGE SPECIFICATIONS  table. If voltage is not within specifications, replace CTS. If voltage is within specification, go to next step.
    CTS 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 CTS connector. Using DVOM, check continuity between chassis ground and specified connector terminal. See CTS GROUND CIRCUIT TERMINAL ID  table. If continuity does not exist, repair wiring harness as necessary. If continuity exists, go to next step.
    CTS GROUND CIRCUIT TERMINAL ID

    Application Terminal No.
    All Models 2
  4. On Stealth, and 3000GT, go to next step. On all other models, Disconnect CTS connector and ECM connector. Check continuity between specified CTS connector terminals and ECM connector terminals. See CTS TO ECM WIRING HARNESS TERMINAL ID  table. If continuity does not exist, repair wiring harness as necessary. If continuity exists, go to next step.
    CTS TO ECM WIRING HARNESS TERMINAL ID

    Application CTS Terminal No. ECM Terminal No.
    All Models 1 63
  5. Turn ignition switch to ON position. Check voltage between chassis ground and specified CTS connector terminal. See CTS VOLTAGE CIRCUIT ID  table. If voltage is not 4.5-4.9 volts, replace ECM. If voltage is as specified, condition required to set fault is not present at this time. Test is complete. Intermittent problem may exist. See TESTS W/O CODES article.
CTS VOLTAGE CIRCUIT ID

Application Terminal No.
All Models 2
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