Overhead Door Brand Opener Diagnostic help?

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
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Yep, works the same way. That's how I've been able to utilize the door while this problem exists.
It appears that I may have wiring issues with the side that was not illuminated. I swapped sides and the "red" one now blinks two times. According to the OD troubleshooting that means either the wiring to the other side is bad -or- the sensor is defective. Seeing how when I moved the non-working sensor to the other side and it comes on, I guess its a wiring issue.
But, regarding replacing the sensor, can anyone advise as to what other sensor(s) will work with this unit (Phantom 777)? The current ones are "discontinued".
Thanks
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