the parking sensors on my 2003 X350 (3.5 ltr version) won't stop beeping. I'm pretty certain it's the front rather than the rear that's at fault - how do I access the sensors without taking the bumper off (if at all possible)? My plan is to isolate each sensor in turn (by disconnecting it's supply wire) until the continuous beep stops. I know it's possible to replace an individual sensor, I just need to identify the culprit!

I don't have sensors all around but my rear ones only work when reverse is selected.

hence your front ones will only work when D selected.

You should be able to reach the fronts by removing the two (small and large) under trays

aren't those sensors black in color? Not to be negative, but adding the price and hassle to get those painted to match seems much. I found the exact color match on ebay for a used one and it was around $25. I only needed to replace one.

unplugging the sensors won't stop the beep. turn ignition on apply parking brake maybe even chock tires. place vehicle in reverse NOT RUNNING and put your ear near the rear sensors. you will hear a faint clicking on the working sensors. the quiet one is the bad one. not sure about the front ones as i don't have them but i would guess place car in d and listen to them. car doesn't have to be running and it's probably not a good idea to do with car running. or find someone with a code reader that can do parking sensor system and get the codes.

You can't turn them off with a switch overhead by the glasses case?

Sure, but if they're always beeping when turned on, you can't use them when you want to. My car barely fits in the garage, and I use the parking sensor to know when it will clear the door coming down. I can't say I've ever actually used them out in the world, but they sure help in the garage! And I admit the rear ones saved me on one occasion from a post I didn't see while reversing...

Understood, i like them as well, the fronts are a pain when snow covers them.
As far as the garage space, with motorcycles in the rear of the garage.
You need to take extra care, not to back in too far, have used
A length of wood, a tennis ball, hung touching the windshield.
Telling you, that's far enough, time to stop.
Long before i had a car TELLING me when to stop.