Bill, The 6v does sound good...it should be between 5-6v at the opener. The low reading at the sensors is not good.
Since both sensor lights are out this is saying that the sensors, or the logic board or the wiring to the sensors are the problem. Sometimes the wires get corroded at the connections and you could cut, strip and reconnect the wires to the sensors. You could also have a short if a staple has worn through the wire.
If one or both sensors are blinking, the sensors are misaligned.
If one of the sensors is out, the door sensors typically need to be replaced.
If both sensors are out, the problem is typically a bad logic board or bad sensor wiring.
You need to look closely at the wiring, to the sensors, to see if it they are broken or pinched by a staple. You can test the wires with an ohm meter for continuity or two wires shorted together.
If the wires appear to be good, remove the sensor wires from the terminals on the logic board and check for voltage across terminals 2 and 3 on the logic board. If you have 4-5v you will need to replace the sensors. If you don't have 4-5v between terminals 2 and 3 and the opener is plugged in, the logic board is bad.