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).