P0128
P0128-THERMOSTAT RATIONALITYWHEN 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
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding.
Diagnostic Test
1. COOLING FAN
NOTE: refer to the no trouble code test "*Fan Operation Test" to ensure the fan is operating properly.
Q: Is the cooling fan operating properly?
YES: Go To 2
NO: Repair the cooling fan.
Perform POWERTRAIN VERIFICATION TEST VER - 2 (DIESEL). Powertrain Verification Test - Ver 2
2. COOLANT TEMPERATURE SENSOR
Operate the engine for 10 minutes while monitoring the coolant temperature with the scan tool.
Q: 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). Powertrain Verification Test - Ver 2
3. THERMOSTAT
NOTE: refer to the no trouble code test9*Thermostat Test9 to ensure the thermostat is operating properly.
Q: Is the thermostat operating properly?
YES: Test Complete.
NO: Replace the thermostat. Perform POWERTRAIN VERIFICATION TEST VER - 2 (DIESEL). Powertrain Verification Test - Ver 2