My two inner sensors are faulty ie no ultrasound clicking and here is some info that may help others.

Firstly if you don't have access to ramps run your rear wheels over a couple of bricks, it'll give you a much needed extra few inches room to work.

Undo the four T25 screws from the underside of the bumper and wedge the plastic open by 3 or 4 inches with blocks of wood or similar.

Laying on your back behind and half under the car you should now be able to reach the sensors single handedly, unless you've arms like Arnie.

The sensors are a push click fit in holders which are themselves glued to the bumper plastic.

Feeling around the larger diameter of the sensor you should be able to feel the small holder clip tabs.

Try to push one outwards with one finger while wiggling and pushing the sensor from the outside with your other hand, then pull the other tab and repeat the wiggling. Alternate between tabs until it comes free.

Hooray! Stage one complete.

The next stage is to remove the electrical connection.

The wiring isn't long enough to get comfortable access to each sensor but they should dangle just close to the edge of the plastic that you've wedged open, you may have to release some of the fabric tape binding the wiring together for extra length. In my experience the pressure pulling the wires was enough to tear the tape free so no worries.

Use a small flat headed screwdriver with a long shaft in one hand to insert into the large opening on the back of the sensor to open the locking clip and with the other hand work the sensor free of the connector.

It takes a bit of dexterity but it can be done.

Part number for my 06 1.9 TDI L&K Combi:

7H0 919 275C

£33.80 for 2 on eBay. There were half price sensors from Hong Kong but I just prefer dealing with UK suppliers if it goes Pete Tong.

Hope this procedure helps someone.

When I replaced one of mine I found it easier to remove the electrical connection first whilst the sensor was still held securely.

 

I found a seller on eBay in Poland that sold pre-painted sensors making the job easier still. The price was about the same as from UK sellers.

 

http://www.ebay.co.uk/itm/PDC-REVERSING-PARKING-SENSOR-AUDI-VW-SKODA-7H0919275C-NEW-ORGINAL-/321253117100?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4acc2d8cac

 

I plugged the electrical connector back in after the sensor had been reinserted into the bracket, simplz.

 

I have a couple of front ones to do next, they too look tricky to reach :(

Yes, it may be easier to remove the plug from the sensor while it's still in the holder.

In my case I hadn't dealt with them before and tried squeezing and pulling all ways to release the plug, in the end I had to remove them wired up just so I could get a better look at what held them on.