Parking sensor fault -W163 ML 270cdi ‘04

[Mwap5194]

Help!! My 04 ML270 cdi parking sensors don’t work. I did a diagnostic scan which said front left inner centre and outer faulty so I changed them. Plugged scanner back in, cleared fault, drove forward and they worked. Once. Plugged scanner back in and now it’s saying rear sensors faulty. I’ve ordered more but I’m sceptical. Any advice while I wait for new ones to arrive?!