My 2003 XJsport (70k miles) keeps falling in to failsafe mode. I can clear it by shutting off and restarting, and pressing the cruise control button a few times (once I get the cruise control button to turn on I know that the car will run again).

Car shows a P0121 throttle position sensor code.

Do you experts think I should just go ahead and drop-in a (rebuilt) throttle body/sensor?

Or is there a cheaper/better solution?

 

moved to proper X308 section

 

There's a fault known to trigger a P0121 with a failing brake pedal switch - it's a dual use switch incorporating the cruise too, people have fixed this by changing out or replacing the micro switches from radioshack. It's located at the top of the brake pedal and held in with a small nyloc nut and bolt. Tricky to get to. Flush with wd40 or similar, test the switches with a meter. If intermittent, get them replaced or buy a new switchpack?


More expensive fault is the TPS - these cannot be bought so a recon throttle usually is the answer - to test have you got an OBD reader, scroll to throttle position with ignition on, and very slowly press the accelerator pedal, if the signal hangs on a value, or breaks down, its time for a new throttle (TPS)...

I had the same fault a while ago, changed the throttle - all good...

Hope that helps (and I hope it's the brake switch)

 

While you're in cleaning mode try pulling apart and cleaning the TPS plug + socket.

 

Handy info for me too. Have the same problem also a sports.

 

I had the same problem with the Failsafe message and disabled cruise control. I sprayed WD-40 on the brake switch where the plastic piece extends out to touch the brake pedal. That's all it took and haven't had the problem since. It is worth a try before you do anything with the TPS

 

I dealt with both issues. Brake Switch, AND TPS connector. WD40 in brake switch, and either THIS fix, or change the plug end for the TPS.

 

Thanks everyone for your helpful replies!!!!!

 

My car did something similar, when you engage cruise the failsafe would come on.... Turned out to be a broken vacume line to the fpr..

 

This car is 03 - not 98, big difference in this system. Nice tip and added info though