if this is a new installation, check to see if your sensors wire are not reversed at the back of the unit. older install that just started this... beats me

Check your door sensors. They most likely have been bumped so that they no longer point at one another. They each have a small light that must be lit and not flickering for them to work properly. Tweak them a bit if one is not lit. If this doesn't work you may have a bad sensor.

thanks for responding back. sorry, didn't mention this system is at least 10 years old.

I checked the eyes and they are not flickering. You mentioned that it might be a bad sensor, I just don't understand why it would work going up, but not down. If it were a bad sensor, I would think that it wouldn't work either way. I neglected to mention that when I push and hold the button to close the door, the gear mechanism clicks seven times. Any other thoughts?

The reason it will work going up but not down is because there has never been a child or pet squashed by an opening door. The sensors are completely out of the picture when the door is going up. The clicking you hear might be the relay for the light rather than the gear mechanism. Count the clicks again and if you hear 10 when the door is reversing you have a sensor problem.

You say you checked the eyes. Do you mean you checked the small LED lights on the door sensors? Were they both lit?

Thanks Don.

I checked and you are correct, it clicks 10 times. The light at the bottom of the door on the eyes are a continuous green on both sides, I don't know if I am correct in saying that this is the door sensor. And if so, are there 2 door sensors, and how do I know if one or both are bad?

Do I need to go back to Sears for the part?

Yes those are the door sensors. Do one more check. Hold your hand in front of the lens and see if the small light goes out. One should and one shouldn't. Your main light must be missing or burnt out. Each time you hear that relay click the main light should be flashing. Sometimes the light on the sensor that goes out when you put your hand in front of it will be flickering but is very hard to see. Tweak them just a little and check.

both lights stayed green, there was no flickering. when you buy the replacement sensors, do you have to replace both sensors or can you replace one at a time as needed (to be replaced).

You can only get both sensors unless you can find a shop with some used. It is usually the receiving unit that quits.