Details:
2000 Jaguar XJ8 VDP
P0121 Throttle position error code
Engine goes into Failsafe mode

Extreamly intermittant - Some days it does it non stop, most days it does not do it at all, ofted does it after 30+ minutes of driving.

Thought maybe it was related to running the heater, typically happens on cold wet high humidity days. Latley happens whenever?

Usually I can turn the car off for a short period of time and it resets and is fine. Sometmes it just does it over and ove and can't be cleared.

Had the TPS wiring / pin mis match taken care of. This was one of the service bullitins. Cleaned the throttle body, it really wasn't that dirty.

Tech thinks the sensor isn't bad when he tested it it performed fine.

Any Ideas?

 

Bad ECM ground. The ECM has ground wire attached to stud under the engine management fuse box on the left inner fender. Check the ground wires there and check to be sure the ground is actually making it to the ECM properly. If the problem is intermitent you may have to check the ground at the ECM pin when the problem is occuring to find it faulted. Otherwise install a new throttle body.

 

nice real, thats a new info for me. ive only fixed that with a new throttle body. And where are you? Salt belt, we dont have too many iisues with grounds although I know there alot of different problems that are specific to areas of the country

 

Yes, lots of salt here in Michigan. The first p0121 I saw with a bad ECM ground came to me from another tech in the building. He put a throttle body on and a month later put another service part warranty TB on after he put the THIRD throttle body on and the car came back a 4th time they made me look at it. The ground under the Engine management fuse box actually controls a lot of stuff but the p0121 is the first code to set when there's a problem and once that happens no more driving and no more codes can be set. Took me a day to find it and I've seen a couple others since then.

Reminds me of the '04 XJ with all sorts of bizarre problems on the order. The entire ground stud behind the passenger side headlamp rotted off the core support.

 

Thanks for the suggestion / help, unfortunately I haven't had a chance to check it yet, I've got it in the shop, they are looking at my other issue (P0306), When I called the shop the other day I suggested they look at the ground off the ECM for this problem as you suggested, but who knows if they paid any attention to me. I'll know earlynext week.

thanks again.

 

ive seen the ground problem on the 04 xj's behind the headlights.. good old body shops.. drilling a hole and attempting to make a good ground lol

 

The shop wouldn't say exactly what they found, I pressed for detail and couldn't get much. From the service order paperwork it looks like they repaired the connection they made when they replaced the throttle position sensor connector the last time. I looked at the ground you mentioned, it appears to be in tact and OK. I've driven it for a few days, so far so good. I hope it was that simple. Thanks for the help.

 

Hi, I’m new to this forum. I know this thread is old, but it’s the same problem I’m currently having with my XJ8. I’ve owned it since 2007, currently 134k miles.

I’ve dealt with various faults over the years and I came across this thread as I was researching the recent recurring P0121 fault.

I’ve read this and numerous other 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 the throttle body, 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

 

I had a lot of errors for various electrical codes, I would get intermittent error codes and then eventually something else un related would fail. When the actual thing would finally fail the unrelated error codes would go away. The transmission finally went out which was at the root of all the failures into safe mode. I ended up unloading the car, I loved it but it was becoming a destructive co dependent relationship. Best of luck.

 

Thank you for your reply. I agree the trans can sometimes cause some unusual things to happen. The trans in my car was replaced with a remanufactured unit a year ago (~9k miles) and the current symptoms and P0121 code are completely different then when the trans went south.

Thanks,
Larry