I am having a similar problem with my 2002, XK8, 44,000 miles that means the dash display reads, "Trac not available,Transmission Fault, ASC Not Available, Failsafe Engine Mode". There is no gas pedal response, engine runs rough, so I coast to the side. shut off the engine wait a few minutes and drive on without difficulty. Occasionally I get a error code P1121. Nothing else. From what I'm reading I think it might be a throttle body. They are costly, I see a rebuilt one from somebody in Tennessee with a 2 year guarantee. Anyone have any experience with this company. Please let me know what you think.

 

ah the dreaded p1121 - I danced to that one for a few days, it is indeed the TPS going south on the TBody, problem is the pot tracks wear out, the signal is lost and the engine decides it's failsafe mode.
$3000 for a new one, $1200 for reman, ? for unknown s/h one.....you makes the choice and pays the money - good luck, but I'd try cleaning all the electrical connectors on the TBody before shelling out.....

 

If you elect to get a reconditioned TB you can go to ASI and rebuild yours for $500.00 + a few and get a warrantee this link will give you an idea on how to do it. Link JagRepair.com - Jaguar Repair Information Resource

 

Thanks Gus for your advice. What puzzles me is randomness of P0172 - sometimes it takes hundreds of miles, sometimes it pops twice a month. I checked the TSB link (http://jagrepair.com/images/TSB/XK8/303-)
you suggested, but my engine never stops. And quite often the "restricted performance" warning goes away on its own (only check engine light stays on).

 

I decided to let my shop try the rebuilt throttle body about a month ago for a cost of $1300 installed with a lifetime warranty and so far it has solved the issue 100% and my 2003 XJR drives like new. I am very happy I spent the money and if I was comfortable doing this myself I would have tried it but I am not so I paid for the labor and have no regrets.

 

It took a while to get to the Jag issues. I finally found a friend with the same model 1999 XK8 as I have. We swapped MAFs. It did not help. I am still randomly getting 0172 code.It always happens when coasting to the red light, or when idling at the intersection. AFter I leave the intersection and push the accelerator, the engine coughs once or twice as "restricted performance" does what it should and then it returns to normal mode. But recently, I have noticed that a gas mileage when cruising at 70mph (mostly on a long trips on interstates) dropped from 27 to 24 mpg.