I switched out my bumper and reinstalled my OEM parking sensors. When I pushed the button to make sure they worked, I got the malfunction light saying to check the sensors.

I checked all the sensors, checked the connections and nothing that could indicate a bad sensor.

They worked fine on previous bumper. Anyone know what else this could be?

 

You'd need a scanner to check for codes beyond just OBD II codes...that should tell you where the problem area is with the sensors.
You can use something like Veepeak bluetooth dongle with a free app like CarScanner to check codes in the entire vehicle.