2004 XJ8 rear electronic module issue

Hi all,

My dad used to drive a 2004 Jaguar XJ8. He passed away a few years ago, and I’m hoping to keep the car for as long as I can, because it’s beautiful and has much sentimental value. We keep it in very clean condition, but it doesn’t get driven much—I’ve been away at college and my mom drives a different car—so it’s a bit worn and has had problems often.

The latest problem has my mom considering getting rid of it, which I desperately want to avoid, though I know it’s a money hole at this point. The car shows a “restricted performance” warning, and the Jaguar dealer told us the rear electronic module needs replaced, but they don’t have any and don’t know where to order one. I’m not super car-literate so I could use some help understanding what this part is, where I could potentially find one (I did find this—does it look correct?), and any general advice you may have on upkeep.

Thank you so much for reading and offering any help at all!

Max, stop what you’re doing. By stating the car is rarely driven you may have inadvertently solved the problem.

The very first thing you should do is put the battery on charge for a full 24hrs, this can be done whilst still connected to the car, then try it again.

Report back on what you find and we can go from there.

Thanks for the response. I should’ve mentioned, we do keep the battery plugged in while the car is not being driven. I have been driving it for the last two weeks or so, and the dash shows a “restricted performance” warning, though the car has been driving just fine for the most part. The dealer told us that this warning is a result of the faulty rear electronic module, and the car will deteriorate without a replacement.

The dealer will say that because he wants your money. Plug a code reader in and see what codes come up. If the car drives ok then it could be something as simple as a split hose or dirty MAF.

I’m not an expert on this generation but I highly doubt the restricted performance message, could be caused by a module in the rear.
So save your money!

You can by small diagnostic readers off ebay for $10 which will read the codes and point you in the right direction. Post the link before you purchase it though.

I have a used module you could borrow for testing, if after trying the above suggestions, you still want to.

Good luck,


Thanks for the message Andy. I did some reading up on recommendations, and found this scanner highly reviewed in tandem with the OBD Fusion iPhone app. I also found this one as a cheaper option, though it’s not quite as well reviewed. How do these look? I appreciate your help.

Hi Rob, that’s very generous. I will be in touch if the other suggestions fail. Thanks!

I myself have the ELM. The key thing is that you need the WiFi version to be able to use it with an iPhone. So read carefully.
It’s a bit temperamental. But does the job,

Yeah buy either.

I got a code reader and here’s what I found:

I’m not sure what to make of it, any help interpreting would be great. Got a bit confused when I googled the codes, I found that P1338 gives “Fuel pump (FP) -feedback circuit malfunction”?

Seams to Be oxygen sensor related from google.

There are 4 of them so if anyone knows which one this is for…

Maybe a failing fuel pump not supplying sufficient fuel pressure?
P1338 seems to be related to the FP