Hi Folks. I do not intend this info to be an easy one size answer for this code re the trottle body/throttle pedal but I thought I'd pass on what appears to have been a solution for mine, 2000 XK8.

I'd get it occassionally and I tinkered with the linkage then with cleaning the contacts in the plugs and it would be fine for a while then pop up again, usually after a good run then repeated movements in traffic. It occurred to me that the spring assembly was not always returning correctly. Why, I couldn't see but finally I sprayed lubricating oil liberally on the pedal cable and around the spring. I worked it in manually and I haven't had a code in a month now. I also returned the cable to a very tight condition rather than a bit loose by watching to ensure that fully off still rapped the butterfly back against the stops.
Hope that helps someone.

 

Exact same problem as You. Only at very slow speed and very intermitten(maybe every 100 miles or so) Same code p0121. TB was cleaned last year and I don't have mismatched plugs. Thinking about sending TB in for rebuild. Let us know if the lubrication was the solution. I have a feeling it's gonna come back, but I hope not! Good Luck.......mick99

 

Hi all:

Had this code and cleaned and made sure tb connectors were tight. No joy on that as it returned. A fellow member suggested cleaning the ecm connectors and check a ground strap under the ecm unit that was sure enough loose. Once I cleaned and tightened it has never returned. This was last year. Also if you have a battery tender it may be a good idea to charge up your battery as its been mentioned that a weak battery can cause these issues. My car is a 99 xk8 convert and a left hand drive. The ecm and ground strap are under the hood by the windshield on the passenger side under the black lid. And my ground strap i could easily turn it by hand. Been almost a year now and its never came back ( famous last words !!!! )

Howard

 

Well, so far so good. Running at varied temps, both short and log hauls.
I'm also parked on a very dirty construction site for most of the days so that can't be helping.