Don329

I didnt note the code number but it read out as Coolant temp to cold for thermostat to regulate engine temp. That doesnt make sense to me. If the engine temp was cool the Tstat wouldnt need to open, so Im not understanding this. I cleared the code. Can anyone enlighten me?

6.7L MegaCab

From what I read it could be that the engine isn't warming up as quick as it should be. The ECM is probably expecting a certain temperature by X point in time and if it doesn't see that temp, it sets a code. Culprits could be coolant temp sensor, IAT sensor, and something else that I can't recall. Those are generic, not specific.

It would be really helpful if you can post the specific code. What year/model is your truck (can't see signature block in mobile app)?

BigIron70

Usually T-stat will cause this code to set.

Don329

I have a 2012 3500 the code popped while towing. Engine temps were fine so Hell I dont know what it was

Don329

The code came back its P0128

6.7L MegaCab

P0128-THERMOSTAT RATIONALITY

For a complete wiring diagram, refer to the Wiring Information.

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

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.
•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.
•Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).