Problem with rear parking sensors but no error code

[Bund]

I noticed a good time back that my rear sensors were not as sharp as my front or some areas were not picking up objects, luckily through just thinking im to close. I've done some of the DIY checks and all sensors tick or buzz.

Its been into an Audi specialist who cannot find a fault code but have suggested it needs 4 new sensors and is a timely job as has something to do with getting to the bose too. I'd love this to be a load crock as I haven't found any posts where a cars needed all 4?!?! A friend said they're good and as a favour to him they had a look. I'm going to get a second opinion, the parking beep was coming on very late or not at all at the end but never a long continuous beep.

Anyone had all 4 go?

Thanks