2001 XJR: P0128 "Coolant Temperature below Thermostat Regulating Rate" It was obviously running too cool after morning startup. I am guessing I need to replace the thermostat. Do any of you agree? If not, what else should I check? Where should I buy a new thermostat from? Or will any one that matches be OK?
Thanks,

 

Very important to check coolant level. If it falls below the sensor you could overheat and not know. Did the temp needle move to midscale? Assuming the coolant level is okay, replacing the thermostat is the first thing to do. Buy one from any auto store, but be careful you replace exactly in the orientation of the old one, and do not overtighten. That old plastic housing is easy to break.

 

Provided P0128 is the only code, then it has to be Thermostat.



(click on the image to enlarge it)


As advised by RJ237, thermostat orientation is important. The 'giggle pin' must go at the top.



This allows air to bleed and some coolant circulation if the thermostat should stick.

Graham