A3 reversing sensors

Hi all, i think i have a problem with my reversing sensors? when i get in a select reverse a get a continuous beep beep beep.....
then it goes off, after the car has warmed up i turn the ignition off tart it again select reverse and i get no beep beep beep?
seems to be only when cold? they also work ok when i'm reversing and something is behind.

I have vag com and have scanned for faults and dont get any, but when i try and select the parking air controller it says not found?

any ideas appreciated

thanks