Guys,


Last month, three days after having a shop install a used engine, new thermostat, new radiator, and new water pump, the check engine light came on and it was code PO128 "coolant thermostat temp below thermostat regulating temp." I called the shop to ask if they might have put in the wrong thermostat (too low of temp. range) and they said "no way, we got the thermostat from the Jaguar dealer." I cleared the code and it took 4 weeks (only driven 300 miles) but it has come back. One other bit of info: when the check engine light came on 4 weeks ago it was after driving for several minutes, today it came on after the car was move 50' in my driveway and it had been idling for an additional 1 minute, 2 minutes tops.
The parts store, where I had the code checked, thinks it could be a sensor. What say you guys? I'm leaning towards replacing the sensor myself (hoping it is an easy job), and if I get the code again I will go back to the shop that did the engine swap if it points to the thermostat. Can it be anything else?

 

Well, Sorry to be late in responding but after no replies to my question I ordered a replacement coolant temp sensor first, only to have it come in and be the wrong part. Planned to go to the Jag dealer (1 hour from home) but in the two weeks of waiting on the ordered part and finding time to get to the dealer, the problem didn't re-surface. So, I reset the codes and have not seen it since. Sure it is warm now but we did have several cool days in February & March and no re-appearance of the code P0128. Being this a recently swapped engine and I had them install a thermostat and water pump, maybe a bit of debris (gasket material) was caught in the thermostat and has worked loose. If so, this is one for the record, for I always tell my wife "car problems don't fix themselves, they get more frequent and louder, rougher, worse, and then it's usually obvious what wrong."


Tailwinds,
Charlie
2006 S-Type 3.0
Covington. LA

 

About $65 from them, $12 from Autozone. (Just did mine a few months ago)

But if they just put a new one in, I too would doubt its a thermostat issue. I mean it's always possible it was a faulty unit to start with but unlikely I'd say.

 

Maybe fitted the wrong way round? BTW, new ones can be faulty, though not usually.

 

What does your temp gauge show? Even better, what's the water temp when read by an OBDII reader?
I had that code on my XJ8, and it was obvious because it always ran cool on the highway. A new thermostat cured it.