This is my first Jaguar with a V8 i am learning some of the mechanical issues with it (2007 XJ8 VDP 103k miles)
About 2 weeks ago the engine light appeared on the dash the car was running fine smooth with no misfire the gas mileage stayed the same
I took the car to a very good Jaguar shop (Auto Doctor in Southfield MI) they hooked it up to their Autologic and there were 4 codes stored in the ECU, from ignition coils, causing a misfire to Catalytic converter, one of the codes was a thermostat
we decided based on Bill horning's recommendation to start out with a thermostat
i ordered the part and wad was planning on doing it over the weekend and as i was driving the car the following morning i noticed that the temp. gauge was lazy coming up to normal and it took about 5 miles to get to normal
when i took the thermostat out i noticed a small crack inside the plastic housing i called Bill and he confirmed seeing it before on other cars and he recommends changing the housing and the thermostat as an assembly as when the plastic housing cracks internally the hot water is being by-passed acting like a bad thermostat
i did replace the complete thermostat and housing and now the engine comes right up to operating temperature like it should be

 

Yep it's a common recommendation from techs. As the housing is plastic and subjected to incredible heat, it makes sense. Frankly a bit impressed yours lasted that long. Glad it's resolved.