I Got my 2002 XJR100 back in November 2021 from a service to fix several vacuum leaks around my throttle body intake tubes and supercharger actuator...(causes restricted performance lights etc). The mechanic had to do several "smoke tests" to identify all the leaks. Anyway, I was joyous to finally be able to smash my pedal to the floor and take off like a rocket again with NO CEL codes popping up! But alas, my check engine light came on again last week. The OBD2 reader said P0128, engine too slow to rise to operating temperature. I got a Murray replacement from Advance Auto for $12 and put it in, took 15 minutes. I carefully inspected the old thermostat (appeared to be oem) and noticed the small rubber o ring in the center was deteriorated and gummy, which had allowed Coolant to pass thru when cold, delaying warm up and kicking off the code. Easy peasy fix. The best part is that after about 20 or so start ups, the CHECK ENGINE LIGHT WENT OFF BY ITSELF! Which is what I was hoping for. Oh happy Day again!

 

I am not getting any trouble code but I am finding my auto control does not want to switch from AC to heat now that it is winter. This has happened in past years but then after a long drive the heat suddenly starts to work and it is good all season. Probably not the thermostat like yours. I'm thinking a sticky valve but I should probably change the thermostat too.

 

Elmo, I would change out your thermostat first as it could be contributing to this issue. If your engine is not warming up to operating temp due to cold coolant bypassing the thermostat, it could explain why your heat doesn't work until after a long drive. Easy fix. Get the correct temperature thermostat for your car and a container to catch the coolant. I like to use those plastic quart containers that won-ton soup comes in. You can bend them to fit right under the housing and catch every drop. Have 2 or 3 handy. For $15 or so you can eliminate the thermostat as being a contributing issue.