2 blinks of the LED on the rear of the motor unit means "sensor shorted or wires reversed."

Don, Thanks. I'm glad I was thorough in the description.

I think I can rule out "wires reversed" because they were not touched when this occurred, and I have verified that they are located where, according to other messages, they belong--red stripe (from wall switch) to pole 1 (red), three white (from wall switch and sensors) to pole 2 (white), and 2 black stripes (from sensors) to pole 3 (black).

To try to locate a short, I disconnected the wires to the sensors from the unit then sent power to each (independently, using what I have access to--an AC/DC adaptor--to control voltage). The sender lights up green. The receiver will flash green, but I can't get this to happen consistently. Reconnected to the board, together or independent, neither sender nor receiver lights up, and the problem persists. (And in the midst of this I got 4 flashes from the unit warning light.)

I infer this to mean one of two things. First, there may be a short on the board, since I can get the sender to light up with a different power source. But putting a meter on the two sources (unit and AC adaptor) shows only about a 3V difference, with more from the unit. Trying to rule out some sort of shutdown or load issue when the sensors are attached, I do find more of a drop with the unit (7 to 2.5V) than the adaptor (5 to 4V).

Alternatively, maybe there is something amiss with the receiver. But if so, why would the sender not light up from the unit, regardless?

The puzzle continues ... I'd be grateful for any further thoughts.

As you well know both sensors are connected to the same terminals. If the receiving unit is shorted it could cause the light in the sending unit to not light. Try disconnecting the receiving sensor at the terminals and then see if the sending unit lights. Won't fix anything but will verify our diagnosis.

Don, Yes, I tried connecting the sender and receiver independently to the terminals (i.e., one at a time with the other completing disconnected). No light comes on in either case.

Unfortunately there is still the possibility that it is the logic board and the fact you had s storm pass through almost cinches it. The components on that board are sensitive to surges. Leaves us still in the air and I can't help you with the voltages as I never ever had a reason to check them. If you live in a large enough area you might call around and see if anyone has a used board they will sell you. They change these openers out on a daily basis.

Thanks Don. You have been very helpful. I'll seek to swap out the board, and I'll post the results so that others might learn from the experience. Best wishes.

Don (and others), Here are the results.

I had things narrowed down to the receiving sensor or the logic board. On the promise of a return if parts were in shipped condition, I ordered the sensors, which come as a pair, and the logic board. I wanted to be able to test out both quickly and be done.

Attaching the new sensors directly to the unit, I found everything worked fine. So, it was not the logic board. It had to be the receiving sensor so I replaced it by splicing in at the door. And the opener still did not work!

I started down the list of possibilities again, for example, I considered whether it might be just a short in the wire to the receiving sensor. But to test that I'd need new wire.

Before running for wire, I decided to make absolutely sure other possibilities were eliminated. If the new sending and receiving sensors worked at the unit but not at the door, the problem had to be a wire.

I swapped the new sending sensor for the old one. And everything works fine! In other words, the problem was (at least) the sending sensor, even though that sensor would light up from an external source which had a slightly higher voltage.

An interesting puzzle solved. Thanks again.