P0128-THERMOSTAT RATIONALITY
Theory of Operation
The thermostat rationality diagnostic runs during a drive cycle that starts with the coolant temperature below 140°F. This diagnostic looks at the how cold the system is at start up, what the inlet air temperature is, and the amount of fuel burned during that drive cycle. The PCM estimates what the coolant temperature should be. If the coolant temperature is not above a calibrated threshold, the rationality fails. The PCM assumes that the thermostat is stuck open.
When Monitored:
While the engine is running.
Set Condition:
The coolant temperature does not rise above a calibrated amount over a calibrated amount of time.
Possible Causes
COOLANT LEVEL
COOLANT TEMPERATURE SENSOR
THERMOSTAT
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
1. CHECK THE COOLANT LEVEL
NOTE: If DTC P0116 is also present, repair the P0116 DTC before proceeding with this test.
NOTE: An engine block heater or oil heater can cause this DTC to set erroneously.
1. Turn the ignition off.
2. With the engine cold, verify the level of coolant in the cooling system. Is the cooling system full of coolant?
Yes
• Go To 2
No
• Fill the cooling system to the proper level.
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
2. CHECK THE COOLANT TEMPERATURE SENSOR
1. Start the engine.
2. With the scan tool, monitor the coolant temperature for 10 minutes. Did the coolant temperature change over time while the engine was running?
Yes
• Replace the Thermostat in accordance with the Service Information.(Refer to 07 - Cooling/Engine/THERMOSTAT/Removal)
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
• Replace the Coolant Temperature Sensor in accordance with the Service Information.(Refer to 07 - Cooling/Engine/SENSOR, Coolant Temperature/Removal)
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).