Parking sensors fixed but how?????????????????????

Since I bought the vehicle the parking sensors were very random, most of the time oversensitive and false triggering, often sounding continuosly as soon as reverse was engaged and occasionally doing what they were supposed to, or so I thought.

Then I had a Romanian car wash, you know the one where 20 of them attack it in one go, they used a very high pressure washer with traffic film remover which took all the paint off by towbar bumper guard. But then I found the reversing sensors were working properly, better than they ever had done and repeatedly, it was mended!!!!!

That was maybe 6 weeks ago and they had never played up once, 2 days ago I had a spirited cross country drive with a few light suspension bumps, then the parking sensors were just making a continous tone.

Checked the ultrasonics by ear, all working, also found that I have lost hearing in that frequency on my left ear 😧 I had tried cleaning them by hand, giving them a whack etc and nothing changed

Today I got the pressure washer out and used a pencil jet on all 4 of them thinking i was wasting my time and to my surprise they worked again immediately after.

So what the hell have I actually done and has anyone else had this problem?

When they were partially playing up it was like the ultrasound was being directed downwards or perhaps diffused because they would trigger when I approached my very shallow parking ramp or the very shallow half kerb beside the car.