If memory serves, about 4 to 5 volts DC between sensor terminal 1 and 2 and about 25 VDC between terminals 2 and 3 at the control board at the opener. I had a similar problem a while back and replacing the 2 sensors worked for me.

I have never checked my manual and don't know how these systems actually work as wprice appears to be familiar with. However, my light will blink like that (door won't activate or will start, then immediately stop) WHEN the two send and receive devices beside the door bottom aren't well aligned. I find that bending the one with the potential green LED light until the LED IS solid green solves the problem.

To add, one sensor led should always be on and the other will blink or go out when it does not receive the signal from the other sensor. My problem was that both sensor leds were always on regardless.

Thanks guys. being i replaced the opener a few years ago but kept the cutoff lights, i had a new set in the attic. Upon testing, i determined something about the sensors. After about 45 min. i found that the wire in the walls was shorting. Go figure... Thanks again.

Glad you figured it out. Was going to chime in and say if all seems to work but it won't shut, it's the safety sensors 90% of them time... other 10% is something is binding or the tracks are moved from someone hitting it or wheel coming out of the tracks etc.

I pulled out the set i never used and connected it to the opener then ran it confirming something wiht the sensors, so i picked one and replaced it- still no go. so i replaced the second one. So tested continuity and found the problem. To think- the wire sitting in the walls "went bad?!?!?!"- literally.

On this note, my car has vacuum problem and it looks like the problem is the block of aluminum throttle body- how the air passage in a block of aluminum "breaks" u got me. everything else tests find and plug into this... so like the wires in the garage- the part u would never think would "break".