I'm posting this to encourage anyone facing this "daunting" task to go ahead and give it a go. I replaced the O2 sensor on my '90 Majestic yesterday without difficulty.

How I did it:
Place car on jackstands and standing on a step stool;
Working from above place your O2 sensor socket, using appropriate wrench extentions, in position over the sensor, climb under the car and check that the socket is correctly placed/lined up on the sensor. Return to your wrench above and break the sensor loose and remove.

The new sensor can be turned into its mounting hole from under the car. This part of the job is a little "fiddly" but not difficult. Use your socket from above to torque the O2 sensor to spec and you're finished. I don't think the entire job took me half an hour yesterday.

My MPG went from 15.4 miles per gallon to 18.5 miles per gallon. I expect further variation on my mpg and I will post any changes here.

 

Hi Roger,

Thank you for the report on your successful O2S change. Like you, I found that working from above with a long socket extension was easiest. I posted some photos at the links below (the large images are currently down, but the thumbnails and descriptions are viewable):

Jaguar XJ40 O2S Replacement Part 1 of 2
Jaguar XJ40 O2S Replacement Part 2 of 2

Cheers,

Don

 

Good morning Don, I reviewed your post on the "other" Jag site before I started.
One tip I left out of my original post is that I wrap duct tape around my socket swivel. The duct tape makes the swivel less "floppy" and easier to maneuver, while maintaining flexibility.