I've had this opener for a number of years. Its the Phantom belt
drive model. Always worked like a charm. The other day, however, it
won't close. The opener is in the open position (door disengaged)
and when I press the wall button it starts, moves about 3 inches then
stops. If I press the wall button again, it moves another three
inches then reversed back to the start.

So - what should I check?

The first thing I did was clean the optical sensors near the door
bottom and aligned their beams. The are clear and pointing right at
each other. Of the two only one light, the green, is on.

Of course I'll now do some end to end electrical testing but I thought
I would post this Q to see if anyone has any "try this" ideas.

Thanks