2009 4th gen ram 1500 with 5.7 engine. 55,000 miles. Backflushed cooling system with thermostat removed,replaced thermostat with oe part,refilled coolant with 50/50 mix extended life coolant and replaced radiator cap. All of this was done for maintenance reasons with no prior problems. Now i have a check engine light with code po128 thermostat rationality. Have cleared code and it comes back. Checked evic display and coolant temp staying at 165 degrees at highway speeds. It also takes a while to get to 165 degrees. I checked with dodge and they do not make any other stat for this engine also no service bulletins. If any one can help i would appreciate it.

 

Sounds like a bad thermostat. Oem should be running at 190+. I would say you got a bad stat.

 

Found this:

Some customers may complain of a MIL illumination. During diagnosis, DTC P0128 may be stored, indicating that “thermostat rationality has been set.” This condition may be caused by the block retaining feature for the thermostat not holding the thermostat in place. This causes the thermostat to tip into the engine block opening, allowing coolant to pass through without going through the thermostat. With the thermostat out of position, the system will set this DTC.

 

Sounds like its sticking open or not fully seated in housing.