[villagetinker]

Originally Posted by jayerose

One sensor has a red light; the other is not lit...should it be green?

The color of the LED depends on the make of your garage door opener. If you have a MVP opener, one of the sensors has the RED light the other does not have any light. With the garage door UP you can block the beam and the RED light will go out and come back on when to remove the block. On the back of the MVP garage door opener there are 2 LEDs one RED and one GREEN. Normal operation both are off. If one or both are flashing they are displaying a code for a problem. Your instruction manual has the listing of the codes. I believe I have this downloaded if you cannot find yours.
Also, if the door is up and starts down and then goes back up you probably have a loose wire at the connection of the sensor to the cable going to the opener, I just had to fix one like that last week.
Hope this helps.