Parking sensors working intermittently

[Oopsitsbrokenagain]

My parking sensors are working very intermittently, more often than not when I go into reverse there's a loud and constant beep for a couple of seconds and then it stops, when it does this, the parking sensors do not work. Sometimes when I go into reverse there is no beep at all and the parking sensors do not work. And very very occasionally I get a small singular beep when I go into reverse and this is the only time the sensors work. 

Does anyone know what the cause of this might be? 

I drive a 2007 A4 Avant S-line

[Magnet]

Hello Paddy,

Thanks for joining and posting your plea.

As you probably know, the ‘signal’ that the reversing sensors are going to work is a short bleep when you go into reverse. A long 10 secondish bleep when selecting reverse indicates a fault. 
When you don’t get any bleep when selecting reverse then you suspect the reverse switch itself. So, first thing - are the reverse lights working every time? Please report back on this. 

If they are working correctly then two possibilities:- one or more faulty sensors in the bumper. Some say you can hear the good ones buzzing when in reverse, or indeed feel them vibrating, so you might be able to find the faulty one out of these 4 -? . That hasn’t worked for me! 
Second possibility is the reverse parking module which usually lives behind the o/s boot side panel. 

How do you fault find? ( assuming you have tested that the reverse lights are operating). Diagnostics I’m afraid, unless you just throw a full set of sensors at it and hope! Diagnostics will pin point the actual sensor at fault, and the system seems to work on one-out-all-out.
New sensors -around £160 each from Audi ( no the 0 isn’t a typo!). 
eBay ( try Parkingsensors Direct) - possibly less than £13 each, and you will probably find they will be embossed with the actual Audi part number! 
Oh, you will need the part number. Those doing the diagnostics should be able to give you that.  
Bad news:- likely the bumper will have to come off if it’s a centre  sensor at fault. 
Let us know how you get on Paddy.

Kind regards,

Gareth. 
p.s. some advocate a good spray dose of WD40 into each sensor. Wishful thinking, but worth a try. 

[Magnet]

Hello Paddy,

Just wondered if any of the advice given to you is, or will be of any help? We notice you have returned to the forum and no doubt have read it. 
Kind regards,

Gareth.