I was admonished to move this from an old and "wrong" forum, so here goes.

Good morning Brutal--My Jag is a 2006 S type with 4.2L and this code came on--my local mechanic (not dealership--none within a couple of hours drive) changed out the thermostat and housing and next day, same code again. I'm in Louisiana so same temps as Houston pretty much. Any ideas?

Thank you.

 

Telling us the code would help.

 

Silly me...it was in that thread I was chased away from. P0128

 

When DTC P0128 appears with the 4.2 litre-engined cars, it usually indicates the thermostat seal is leaking causing the thermostat to take additional time to open.

It is a good idea to replace the water outlet pipe, which contains a new thermostat and seal in situ, each time the coolant is replaced. The JLR part number for the water outlet assembly is AJ811793.

 

Howdy NBCat and thank you. Given that a new thermostat was just installed, is this still a likely scenario? Would the water outlet pipe not have been replaced with the thermostat/seal?

 

The water outlet is made from plastic and can only withstand a finite number of cold and hot cycles before failing. Quite often someone will replace the thermostat without replacing the water outlet housing assembly.

If the water outlet fails, it can cause catastrophic damage to the engine.

 

Thank you for the info and I'll look into that possibility right now.

 

Well phooey. The shop changed out whole unit (AJ811793) so the water outlet pipe with seal are new. No evidence of leaking coolant at that location. I'm open to the next suggestion to check.

 

How many start/stop cycles has the vehicle been through since the water outlet has been changed?

Was the DTC erased at the time the water outlet was changed and has now come back on?

Remember, you are not looking necessarily for external leaks with P0128.

 

Howdy NBCat,

DTC was erased after thermo/outlet/whole shooting match was changed and then code came on the next morning again after about 10-12 minutes of driving time. Reset a 2nd time after mechanic gave it a quick look, found nothing and assumed it was just a recurrence of the old code. Then came on again today, so 4+ stop/start cycles now.

I love a good mystery.

 

If the water outlet was indeed changed, it does come with a new water temperature sensor as well as the thermostat and seal.

Check the wiring to the sensor and be sure the system has the correct coolant level and has been properly bled.