1-you need to check the connector on the sensors are you getting power to it, did the wire to it got pinched at the rear bumper
2-and also have the park aid modules scanned to retrieve fault codes , to know what is failing , most cases a sensor will be failing intermittently
3- you can try cleaning them with a wipe from outside
if it is the first time it happens , you can try resetting it manually if you dont have a ford scanner or equivalent
by this below procedure ,But a scanner is recommend , doing it manually ,you will loose the radio ,so will need to make sure you have the security code for it in manually after you connect battery .
Master Reset Procedure
1-Disconnect battery ( Both Cables ,The Negative & The Positive Cable )
2-Touch Cables together and hold 60 secs and let go
3-Hold Brake pedal down 10 secs
4-Wait 15 minutes and then Reconnect Battery
5-Close all the doors ,hood & trunk .
6-lock the car with key from driver door ( IF your car is Equipped with a bladlless key then just locked with the keyFob
7- Unlock car & Start your vehicle & check the parking aid
Please let me know if you need anything else on it, just reply back & we will continue
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ASE Certified Technician