Hello all,

I have a 2005 S-type R, and am having a somewhat strange problem. I am consistantly setting a P0128 code, and so far have replaced the thermostat once with an aftermarket part, when that did not cure the problem, I pulled it back out and put one in from the local Jag dealership.
I still have the code returning, so I looked at the coolant temp sensor, and found it was reading between 64-70 degrees C, which is about what my dash temp readout is showing as well. I have checked the engine's actual temp with an infared thermometer and it is between 120-140 degrees F.

I went ahead and tossed a new coolant temp sensor in it, (the one at the crossover pipe near the supercharger) just as a "why not" and my code is still returning.

Has anyone run across anything else that will set this code so I can inspect? I am running out of ideas.

Thank you,
Chris Williams

 

Well you have done what is called for and that's replace the thermostat. It has been reported that the rubber surround splits and coolant then can bypass the thermostat before it opens.

Brutal one of our Jaguar techs on this board has suggested changing the thermostat too. He uses the Jaguar one and NOT aftermarket but you went back to the factory one with the same problem?

Only thing I can think of is to replace the thermostat again with a factory unit. Might check that it is operating correctly before install? You can heat up a pan of water and then dip the thermostat in to check when it opens.

Just a check but you know the thermostat must be clocked correctly? Jiggle valve at the top?

Oops! One more question? Were the codes cleared properly and completely each time?
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May I inquire if you are physically clearing the code after each attempted repair? This particular fault (like a misfire) may not be 'self clearing'. Just checking before advising further.

 

Thank you for the replies. I have not tested the thermostat in a pan of water, I will do that this evening. I did install both with the jiggle valve at the top.

I am clearing faults each time and running the car through a drive cycle to see if the fault returns. If I drive the car twice through two cycles, I do get the service engine soon lamp on the instrument panel again.

I will report back on the thermostat test and at what temp it opens in the pan of hot water.

 

The dash reading (such as via ETM) is from the same ECT sensor.

If it's still very low now you've changed the sensor, I reckon you're looking at a bad connection between sensor and PCM (power or ground). Or it's not sitting in the coolant (or it's not flowing right). Bleeding air out can be tough so although I doubt that's the problem, maybe it is.

It's hard to see it being both thermostats.

 

Agree, two thermostats in a row failed? Let her warm up, turn the heat on full blast, and remove top from expansion tank...Letting trapped air out.

 

I tested the thermostat last evening, and it is begining to open at 179 degrees, and is fully open by 184 degrees F. This appears to be pretty much perfect.

I am going to attempt to bleed the system again, and will also check the signal for the ECT sensor again. My issue with it not being the ECT sensor is that according to the infared thermometer, the engine actually IS running too cold, and shows about the same temp that my scanner is showing the ECT to pick up.

 

Not sure an IR is trustable that way. Could you check the actual coolant temp maybe?