Hi, I’m new to this forum. I am re-posting this as a new post in hopes of getting more response/help.

I'm currently having a P0121 TPS issue with my XJ8. I’ve owned it since 2007, currently 134k miles.

I’ve dealt with various faults over the years and have had pretty good luck correcting past issues. This is the first time this code has come up on this vehicle.

I’ve read numerous related threads on this forum, plus the factory service manual and the TSB regarding the tin-plated TPS connector pins.

I’ve only done a few basic things thus far:

1. Initially there was no code or fail safe mode, just would barely start or run. Had my local Jaguar mechanic look at it and he cleaned the throttle body, tested and “reflashed” the TPS (lasted 2 weeks).

2. The car was fine until the fail safe mode and P0121 came up suddenly yesterday. I checked the TPS connector pins and mine are tin plated. This is a So Cal car and there are no visual connector pin corrosion issues at all. And the connector is in good condition with the stays intact.

The service manual says to check the circuit for high resistance, high voltage short, and short to ground. If none of those are the issue, then replace the TB. The manual doesn't address the more common issue with many electrical faults, being poor connections.

There have been many suggestions on this forum such as changing to the gold pins, cleaning the connector, replacing or rebuilding the throttle body, replacing just the TPS, and tightening the ground under the ECM.

My question is, have any of these fixes worked long term? There doesn't seem to be much follow up as to what has solved the issue or not?

Thanks,
Larry

 

Update since my first post. I’ve tested the circuit per the factory service manual and the harness resistance is in spec, and there are no high-voltage or ground shorts. With those results, the manual says to replace the throttle body. Given the cost of a new TB, that is my last resort.

Despite the lack of any visual issues with the connector pins, I cleaned them anyway. I will first see if that cures the problem.

If the problem comes back in a short time, I’m thinking the connection is likely not the problem and the TPS/TB may be my only option.

If the problem comes back after an extended period, I’m thinking of installing the gold plated connector pins. The thought being the connection may be the issue.

I’ve considered a used TB or just a used TPS, but they both seem like a crap shoot. I’ve also considered having ASI rebuild mine, but that’s a little pricey and being on the opposite coast from ASI, the car would be down for an extended period. Plus, with new TPS’s not available, I doubt I’d gain anything from ASI’s work if the TPS is the true cause of the intermittent problem. I don't believe ASI has new TPS's to install when rebuilding.

My Jaguar mechanic tested my TPS with the factory diagnostic tool and it tested good. He said that doesn’t guarantee it’s not an intermittent issue with the TPS.

Does anyone have feedback on what has actually worked, and what hasn’t, to resolve the P0121 TPS code errors?
As always, any feedback is appreciated.

Thanks,
Larry

 

Larry - I had the same problem and just decided to bite the bullet and replace the throttle body. That was a year ago and so far no recurrence. There is also a TSB on this issue:http://jagrepair.com/images/TSB/XK8/303-58.pdf which indicates a mismatch of electrical connector terminal materials. So if you follow these instructions it might save you some cash. Steve

 

We had the throttle body rebuilt by ASI in August 2012 (110,000 miles).
No problems with the throttle body since the rebuild.
2001 XJ8 current mileage is almost 139,000.

Look at this thread I posted in August 2012:

https://www.jaguarforums.com/forum/x...rebuild-79462/

Jim Lombardi

 

Thanks for the feedback guys. I think it benefits everyone when there is longterm success posted for common issues.

Steve, Good to know replacing the TB has resolved the issue with your car. Yes, I've seen the TSB on the connector pins and I am considering this depending upon how long my current attempt lasts.

Jim, Good feedback on the ASI rebuild of your throttle body. Thanks.

Since my last post, I cleaned the connector pins with contact cleaner and it's been good for a week now. If this holds up for at least a few more weeks before happening again, I'll probably try replacing the pins with the gold plated parts, per the TSB.

Also, since my last post I've found remanufactured TPS alone for $250 plus core charge. This may be an option, too.

 

I'd almost bet that replacing the TPS will do it. I went through the same problems and potential cheaper solutions that you have, but finally ended up becoming an ASI customer. No more problems.
Cheers!

 

Thanks, Scott. You're probably right, that a rebuilt or new TB will be the ultimate solution.

 

Update. Since cleaning the TPS contacts on 10/5, the restricted performance and P0121 code related to the TPS problem has reoccurred only once so far, on 10/18.


However, the very first symptoms reoccurred on 10/25. That being crank and start, but barely run, with no codes and no restricted performance. This is the same thing that occurred when I first sent it to the local Jag mechanic.


This time I was able to look into it further myself and determined there was little-to-no fuel pressure. After reading the factory service manual, I checked for voltage at the tank connector and it was good. So the fuel pump was the likely issue.


Side note: the fuel pump gets voltage for only 2 seconds in the key-on position. So I had to have someone work the key while I checked for voltage. I also checked it during engine cranking, which should be constant voltage. All good.

I replaced both the fuel pump, pickup strainer, and the inline filter (under the car), as the mechanic had said the filter should be replaced every 30k miles and it had been about twice that long.


So the FP solved the no start/no code issue.


The P0121 TPS code/restricted performance issue has not reoccurred since before I replaced the pump. So the jury is still out on that one.


Though I believe I had two unrelated issues, I wonder if the failing fuel pump could have any bearing on the TPS issue? I guess well see.