Green lights are on both sensors
Door goes down when control button held down
Green lifts stay on when beam is broken
How do I know if a sensor has gone bad?

Make sure the lenses on the sensors are clear of dirt, spider webs, etc. and also see if they are aimed directly at each other. To me it sounds like they are mis-aligned or dirty.

I know of no simple test for a bad sensor.

Good luck,
WallyH