I purchased a running/driving 2005 L300 at auction with 44k miles. When I brought it home, before doing any work on it, it had a check engine light and a code for the engine not coming up to temperature.

The first thing I did was replace the timing belt, coolant temperature sensor, coolant, water pump and thermostat. After clearing the codes, the CEL came back on and the temperature gauge on the dash only goes up about 1/4 of the way. I pulled live data from the OBD-II port, and I can confirm that the engine only gets up to about 185 degrees. The thermostat that I bought is a 198 degree model, which is the factory desired temperature. I can confirm that the radiator fan is not running.

I would think that if the thermostat were the problem, the second one would have fixed it. It seems unlikely that two thermostats would have exactly the same problem.

What would you suggest? Car drives fine, heater blows warm. I just have an annual inspection and the CEL can't be on...

 

Across other brands, the thermostat opening too early is almost always the cause. On the Ford Diesels the thermostats have a rubber seal that starts to come apart and lets coolant circulate too soon. That is what trips the code. My LW300 runs at almost half gauge. The Ford type of failure is what I have seen on my L200s a couple times. My guess is the replacement thermostat is not doing what it needs to do.