2003 XJ Sport. I was traveling over Thanksgiving holidays, cruising at about 80. All of a sudden the dash lights up along with an engine failsafe, TCS unavailable in ASC unavailable. Luckily I carry my scan tool with me so plugged it in, pulled the codes and low and behold it was trouble code p0121 which is circuit performance to throttle position sensor. I was still about an hour away from my destination however the code cleared. As soon as I got down off the interstate though the code popped back up and the engine had very little response to throttle. So I'm thinking oh man here we go $800 throttle body. Problem only happened at idle or when I would take my foot off the throttle. Did a little research and I noticed that the throttle position sensor itself could be ordered no longer needing to replace the whole throttle body. However I was out of town and I did not have access to that sensor nor could I it before I needed to make my 8-hour trip back home. So while on vacation I did a little more investigation, hooked the scan tool up and I noticed that I only had the problem at idle. So at idle I noticed that the absolute throttle position percentage wouldswing as much as 3% sometimes, that's when the code would set. So I took an even closer look and my scan tool, I was able to see the voltage on both tracks of that throttle position sensor. I didn't really see anything abnormal but I did learn something. So here's my theory, the ECU checks for correct operation of the throttle position sensor by monitoring the changes in both of those tracks. In other words when I would apply throttle one track would go up a voltage while the other track would also increase in voltage so I'm assuming that the computer is looking for an increase in voltage at the same rate in both of those tracks. And since the sensor has to be programmed into the throttle body one would think that you would be able to adjust the throttle position sensor within the range operation but out of the dead spot that was causing the fault code to begin with. So that's exactly what I did, I hooked up my scan tool, tweaked the throttle position sensor and I moved it a very small amount forward changing it's new idle position. Did a reset, programmed in the new position of the old throttle position sensor and voila no codes. I do have a new throttle position sensor that I received but I'm interested to see if this fix actually works. What do you think? Am I far of base or close here.

 

Interesting thought - eventually the tracks wear out as you may have researched on here, once the tracks are worn faults appear, if you've moved the starting point away then you're out of the danger zone, in theory.

It's a very delicate process shifting the TPS, particularly with the engine running. What scan tool are you using? Also miles on the car. ECU doesn't check, it relies on those signals to be constant variables for mapping.

Let us know how the return trip went.

 

Ecu does compare both tracks and reject the other one wich is detected as falty and result is cel light and failsafe mode. Redundant and failsafe is the reason why there is two tracks. If one moves tps out of the worn idle area of the tracks and re learn the tps tracks to ecu. It will work if the voltage is still in the specific area and any of the tracks is not broken elswhere.

 

I would also check the PPS, the pedal position sensor. That's on the other side of the TB where the throttle cable hooks up, and on my car it's the one that has always been the culprit. I have two spare TPS's in my parts box and I'm on my last spare PPS, which is no longer available unless you get a new TB.

 

Yeah that's exactly what I was thinking so the dual tracks are redundant I would assume for a safety measure or diagnosing failure in throttle position sensor since it's a drive by wire. So yes I have moved it out of that trouble area and I haven't had any problems in about 2 weeks. I do plan on putting the new sensor in but I wanted to share this information just in case somebody was in a pinch and the problem occurred at idle. Thanks for your response.

 

The trip home
​​​was okay and did it a few times even with the cruise on if the cruise closed the throttle. But I made it home safely. I didn't do the adjustment of the throttle until I got home.
I'm using a foxwell nt809 bidirectional scanner. Man that thing is worth its weight in gold. The car has 120,000 miles on it. Thanks for your response.