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P0128 Thermostat Rationality

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Mopar1973Man

By Mopar1973Man

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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