I changed my front middle PDC sensors, as I got random beeps without obstacles being present.
Now the beeping seems to have stopped, but here's what makes me worry:
The PDC on the edges show constant signal when I move closer than 30cm.
The new PDC-sensor never reach the state of a constant signal, even if I put my hand directly over the sensor.
Both new PDC-sensors behave the same.
Are there further steps required ?

 

Sounds like all is well to me, any closer than a foot till you hit something is close enough.

 

My problem is , that I cannot tell, what is the distance.
The middle sensors produce the same beep-sequence at 0cm , which the PDCs on the edges would produce on 1m.
So if I wait for the signal to become constant, I'd hit the wall in the middle.

 

Misunderstood, you do indeed have an issue.
Someone with more knowledge than me, should have an answer for you soon.

 

I think I'll have to remove my bumper again and change the PDC sensors again.
I didn't find any hint for calibration , yet.

 

99% sure that no need to calibrate.

The Chinese make PDC sensors now but I do not know if they work ok.

I buy used ones for spares when I see them.

 

I agree with meirion on this. I also have bought used ones on Ebay with no issue after installation.

The actual controller in the trunk could be an issue?

That is what I thought when I had a problem a while ago but it really was one of the front sensors I discovered that after buying a used controller on Ebay quite cheap.