I know I live in Idaho and it's cold up here but today was relatively mild, high 60's. My check engine light came on after a stop light and about 10 minutes of driving. When I got home I checked the code and it's a P0128 which it describes as "Coolant thermostat (Coolant Temp Below Thermostat Regulating Temperature)."

Just yesterday I topped off the coolant with a 50/50 mix of distilled water and the orange long life engine coolant as I was getting the coolant low msg in the message center. I can't help but think these are related.

Any ideas? The car runs fine, does this need a visit to the dealer?

Again, any help is greatly appreciated.

 

initial thought is the thermostat is stuck open and didn't have enough time to 'properly' come to temp before circulating your coolant. that will be more of a serious issue in colder months, so i wouldn't let it go too long w/ changing out the t-stat. It's a common problem, and at least it is open, and not closed..for the moment. But, a problematic t-stat may decide to stick the other way next time, which is 100x worse.

brutal or another tech can probably confirm. We don't hear many of those codes being thrown.

 

Good thought Matt, but the engine did warm up normally and held at the midpoint on the temp gauge. Maybe it was stuck open initially then started working?

Getting to the point where I won't be driving it much if at all during the winter anyway but I'll keep an eye on it.

I've decided to keep my code reader in the car because this car throws more check engine codes than any other car I've ever had. Keeps things exciting! Of course the Jag probably has more sensors than any other car I've ever had too! hehe

 

I've heard British fighter pilots keep an ODBII scanner in their cockpit too! A Jag has similar systems to an aircraft...

 

p0128 is very unusual for your car with the older style thermostat,vs newer with rubber disc that tears or comes loose that causes this code on newer V8s as it sticks slightly open. Now with yours a common trick on thermostat replacement is to pop ofthe brass check ball on the thermostat. This allows a small amount of coolant to circulate and readily allows trapped air out. another important this does is it keeps the lower hose from collapsing and this ends up working the hose off the the housing leaving you stranded. I also have never again replaced a modifyed thermostat again for over heating, but have done the standard ones repeatadly. so if yours has been modifyed like this it may be setting this code on you being in much colder weather than we see in Tx

 

Brutal, thanks for the response.

I don't suppose there is anyway to determine if this procedure has been performed without any disassembly?

I've received the codes on 2 separate occasions now but each time the car eventually warmed up to normal temp which is pegged right in the middle of the temp gauge. I'm keeping a close eye on it but so far it seems to be warming up normally.

 

Hehe... Thanks! Gave me a good chuckle, then I had to wonder... Do they really? You yankin my chain here

 

THE COde is set from it lettingcoolant past too soon. Ive never seen it affect 1 in overheating, and no noticable change to the customer in time to warm up either...but it does keep setting that code till fixed cause the ecu knows the differance

 

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.

 

Or perhaps another sneaky one that is starting to pop up on these 10-15yo cars, and hinted at by OP . . .
It is now known that the coolant level sensor can trigger some weird (and false) faults ie intermittent msgs of "Low Coolant Level" even when topped to brim . . . sound familiar? Some have reported curing by reaching into reservoir and swabbing the sensor tip. However, in view of relatively low cost, majority drain, remove & replace with new.

May not be your issue but, in view of your earlier, repeated and apparently unresolved warnings . . . it's a no-brainer to check.

Cheers,

Ken

 

You've revived an 8 year old post in the XK8 section. Try starting a new post in the S-type section.

 

+1 please do