Parking Aid Sensors - testing

I have a 2013 XF that has decided the parking aid sensors are on vacation. There is a long high pitch sound coming thru the audio system with a warning on the instrument panel. A complete interrogation of faults (Autel) does not indicate any codes from which to diagnose the problem. I have dismantled the front and rear bumper covers, cleaned each sensor and examined for cracks or odd appearance, checked the resistance of the wiring harness front and back, inspected and checked the resistance of the wiring that is routed thru the trunk/boot right hinge, removed the tail trim and inspected the coax camera wiring, measured the resistance of each sensor with the following results.

Sensor / Ground (YE/OG)-Signal /. Power (VT/GN)-Signal
LF. / 5.9 M / 0.712 M
LFM. / 5.6 M / 0.565 M
RFM / 6.6 M / 0.643 M
RF / 7.6 M / 0.560 M
LR / 34.7 K / 0.421 M
LRM. / 28.2 K / 0.406 M
RRM. / 29.7 K / 0.402 M
RR. / 10.2 K / 0.385 M

Does anyone know what the range of acceptable resistance is for the front & rear parking aid sensors?

I am surprised that the ground-signal resistance between the front and rear sensors is so different with the power-signal resistance fairly close. Any information on the proper testing procedure for these “wonderful” (see useless) sensors would be helpful.