I need some advice on my Blue car, 2004 xj8 4.2. not super, serial G31309. I have a code P1646. I have ordered and received the bank 1 upstream sensor for replacement. The job is now in progress.
I have researched and found several threads referring to such a job, one even claimed it was an easy fix from the topside. But none gave any good description of procedure.
So far, I have removed the wiper arms, the plastic cowl trim piece below the windshield, the cabin filter mount box and the lower outer cover for it. As yet I can but barely see the sensors' plug since it is mounted to the body edge. To continue the job in this direction and dig deeper I can see there is a brace holding a large wiring bundle, an aluminum heat shield bolted to the right side of the body (cant see where), the air conditioning lines, and then possibly lastly the exhaust manifold heat shield before I can get at the sensor itself. I am not interested in unhooking a/c lines, and I am not sure how the manifold heat shield would come off/out without the a/c lines being moved or removed.
Does this seem like the right direction to proceed or should I shift gears as it were and lift the car up to try from below?
Barring the lower plastic cover and the front suspension, I am not sure what would be in my way to try from below. I do have a jack adapter to lift the whole front without twisting.
Thoughts or recollections of actual procedures? Thanks!

The area where the cabin filter ought to be (right hood strut at bottom of pic). several parts still in the way without any obvious means of their removal or a promise of ability to remove the sensor.

 

I'm not sure who would have ever told you to access the upstream O2 sensor from the top side. It's readily accessible once you jack up the front end just removing the undertray.

 

Thank you, that's my hope.

 

Did all four on my 04, from the same place, underneath the car.
A hot exhaust, and long handled ratchet or breaker bar, if things get difficult.

 

As others have already said, you get to them from under the car. If they are the originals or have been on the car for a long time they can be a pain to get out. To make life easier, get an O2 sensor socket so you get a full grip on the sensor to undo it. You might also need some heat to free them loose.

 

Above posters sound correct. I replaced the two upstream sensors last year (economy grade). Just a week ago, I started getting another set of O2 codes.

Can anyone tell me if these are upstream or downstream? P0153 - slow O2 response, sensor 1 bank 1 and matching for bank 2.

 

Sensor 1 is upstream. The ECM uses the upstream sensors to monitor the combustion. The downstream sensor is just there to see of the cats are working. If they read the same as the upstream sensors then the cat isn't doing anything.

 

Hey all- You guys are spot on. It took longer to lift the car up and put it on stands than to change the sensor. In my case I was lucky in that since the car has almost no corrosion, even the exhaust its still fairly shiny. As a result the sensor came out quite easily. I have a sensor socket set and used one of them, but I did not need it. It is so Out In The Open that a box wrench would have been easier. I test drove the car about 5 miles- enough to warm it up- and the engine light stayed on so I used my code reader to erase the code. I then drove the car a few more miles and the light stayed off.
With the ease of access to that and, as is pointed out, all of these sensors I have to think that the folks at Jaguars R&D department were asleep at the wheel that day. Add to it the Idea that although the under tray is held on by 7 bolts and 2 nuts they are ALL THE SAME SIZE, I have to think that someone seriously slipped up here!
Thanks for your replies! Problem solved!

The bank 1 upstream sensor at center. The only obstruction is a fairly flexible engine ground strap.

 

Very nice! If you were smart like me, you woulda applied a bit of anti-seize to the threads. That way, it'll be easier to replace your bargain bin sensor when it fails next year