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  • P0128 Thermostat Rationality

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    P0128 THERMOSTAT RATIONALITY

    When Monitored and Set Condition:

    When Monitored: While the engine is running.
    Set Condition: The coolant temperature does not rise by a calibrated amount over a calibrated amount of time.

    POSSIBLE CAUSES

    • COOLING FAN
    • COOLANT TEMPERATURE SENSOR
    • THERMOSTAT

    DIAGNOSTIC PROCEDURE

    NOTE: Refer to VERIFICATION TESTS for appropriate verification test procedures.
    NOTE: refer to the no trouble code test "*Fan Operation Test" to ensure the fan is operating properly.

    1. Is the cooling fan operating properly?
      Yes --> Go To 2
      No --> Repair the cooling fan. Perform POWERTRAIN VERIFICATION TEST VER - 2 (DIESEL).
    2. Operate the engine for 10 minutes while monitoring the coolant temperature with the DRBIII®. Did the coolant temperature change over time while the engine was running?
      Yes --> Go To 3
      No --> Replace the coolant temperature sensor. Perform POWERTRAIN VERIFICATION TEST VER - 2 (DIESEL).
      NOTE: Refer to Symptom: *FUEL HEATER THERMOSTAT TEST to ensure the thermostat is operating properly.
    3. is the thermostat operating properly?
      Yes --> Test Complete.
      No --> Replace the thermostat. Perform POWERTRAIN VERIFICATION TEST VER - 2 (DIESEL).

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