LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Subaru >> 1993 >> Impreza Base >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes >> Diagnostic Code Testing >> Code 21: Coolant Temperature Sensor
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Code 21: Coolant Temperature Sensor

NOTE: Refer to illustration for ECU connector and terminal identification. See 1 in J - PIN VOLTAGE CHARTS article.
  1. Turn ignition off. Disconnect coolant temperature sensor connector. Measure resistance between coolant temperature sensor terminals. See COOLANT TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS table. If resistances are not to specification, replace coolant temperature sensor. If resistances are to specification, go to next step and check harness between ECU and coolant temperature sensor.
    COOLANT TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS

    Temperature °F (°C) Ohms
    68°F (20°C) 2000-3000
    176°F (80°C) 270-370
  2. Disconnect ECU connector E30. Measure resistance between ECU connector E30, terminal No. 3 and coolant temperature sensor connector terminal No. 1 (Black/Yellow wire). Measure resistance between ECU connector E30, terminal No. 11 and coolant temperature sensor connector terminal No. 2 (Black/White wire). A maximum of 10 ohms should be present in both cases. If not within specification, repair harness as necessary.
  3. Ensure coolant temperature sensor circuit is not shorted to ground by measuring resistance between each coolant temperature sensor connector terminals and ground. Minimum of one megohm should be present.
  4. If resistance measurements are not to specification, repair or replace harness or connector as necessary. If resistance measurements are okay, repair ECU harness or connector. If harness and connector are okay, replace ECU.
scientia non olet · About LEMON Manuals