We have a Craftsman 1/2HP 139.53900 GDO that opens fine but when closing it goes down a couple inches, reverses then the light flashes. The door can be closed by holding the button down from inside the garage. The sensors are in perfect alignment. Another symptom is that some times the GDO light will come on for the 4 1/2 minutes then go off all by itself. It does this at all times of the day and night. The green light on back of the unit also blinks 5 times intermittently. I read somewhere that this might be caused by a bad RPM sesor. Could this be the problem or is it possibly a bad sensor, control circuit board or something completely different. The unit is 6 years old and I hate to replace the opener unit unless absolutely necessary.

Possibly more information: The sensors are in perfect alignment. The sending and receiving units' lights are lit solid. I even checked to make sure the wires were not shorted in some way. The blinking light on the back of the unit, and the GDO light going on and off by itself lead me to believe the problem is in the RPM sensor. My main concern is could there be a problem with one of the sensor units themselves (i.e. the receiving unit), or possibly a problem with the control circuit board. I hate to replace a lot of expensive parts and still not solve the problem. Thanks in advance for your help.