My 2004 jaguar xj8 went into limp mode the other day “ENGINE SYSTEM FAULT” “DSC UNAVAILABLE” and “PARKBRAKE FAULT” I pulled over grabbed my trusty scanner from the backseat and it was throwing a P0121. So I cleared the code and the car would be back to normal like nothing was wrong for about 30minutes or so then it would throw the code again but it let me get it home. I ordered the throttle body yesterday when all that happened today it arrived and I replaced the throttle body and now it does it all the time. Somebody help Is there a programming procedure through SSD?

 

I think I figured it out if anyone has the same issue unhook the battery for an hour and hook it back up I can’t believe that fixed it I also reseated the TPS sensor connector

 

The odds are there is a bad ground connection or your battery is discharging. There is something about ground connection to alternator that will prevent battery from charging and/or cause electrical system to seem under 12V.