Just wanted to say Hi from small town texas.

 

Welcome. What you drive? Check out whatever section is appropriate.

 

Welcome!

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Thanks for the welcome
I have 2012 dodge avenger sxt 2.4L
well since you mention it... I am in the market for parts...

 

New to the Forum. Trying to fix a problem with 05 magnum 3.5l. CEL came on Code P0128. Change the TStat. Coolant sensor and antifreeze. Cleared the code. About 100 miles later CEl came back on same code . Temp reaches about a 1/4 way and stops. When driving there have been times the temp gauge drops under 1/4. If Anyone knows how to fix this problem please let me know.

 

Replace your new, failed, thermostat.

Welcome to DF, post up in the magnum section for more help.

 

Per review of the Service Manual for this fault code, this is the list of possible issues. Access to a service manual will provide further diagnostic information and wiring diagram. Hope this provides some assistance for you.

P0128-THERMOSTAT RATIONALITY

• When Monitored: Engine running.

• Set Condition: The PCM predicts a coolant temperature value that it will compare to the actual coolant temperature. A significant difference results in an error. Two Trip Fault. Three good trips to turn off the MIL.

Possible Causes
LOW COOLANT LEVEL
THERMOSTAT OPERATION
SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE
TEMPERATURE SENSOR SIGNAL CIRCUIT OPEN (K900) SENSOR GROUND CIRCUIT OPEN
SIGNAL CIRCUIT SHORTED TO GROUND
SIGNAL CIRCUIT SHORTED TO (K900), (G931) SENSOR GROUND
OTHER POSSIBLE CAUSES
PCM

 

Did you forget to refill your coolant after the new thermostat opened? You could be circulating a huge air bubble in your engine... usually the over flow tank should catch it and add more antifreeze / coolant back in but that is if you made sure to fill that tank. some times you might have to fill it more than once.