Today I disconnected my battery positive for just over a minute.
Afterwards, I found the parking sensors didn't work (they'd been OK not long before disconnecting the battery).
Engage reverse: dasboard PDC switch light gives six flashes, then off, sensors not operative.
Switch on at PDC switch: dashboard PDC switch light gives six flashes, then off, sensors not operative.
Wet sensors are not a factor, the car has been dry for several days.
I locked the car and left it a while, tried again - same.
So I again disconnected the battery positive for just over a minute, then re-connected.
After that, the parking sensors work as normal.
Don't know what the explanation is.
But perhaps this may be helpful to someone. The quick brown fox

Normal practise to perform a Hard Reset is disconnect BOTH terminals, touch them together, walk away for ten minutes and reconnect, this allows everything to reset, perhaps some residual current was still in the system.

Thanks for this.
I take your point about time being needed to settle any residual current.
I hadn't actually been trying to do a hard reset ...
I'd just done the one-terminal quick disconnect since it seems to resolve another (unrelated) problem I have at the moment.
Then that brought about the parking sensor non-operation. The quick brown fox

After a battery change it took half a dozen ignition on/off cycles for everything to come back to normal.

OK , thanks for help.
I've finished messing with the car for a while (hopefully) so I'll try a properly-done hard reset.
Thanks for the info that in may take a while for things to settle. The quick brown fox