Chamberlain HD420EV Garage door opener

[jmm]

Are garage door openers in scope for this group? Apologies if not but I'll explain my predicament and appeal to your better nature to respond or not.

Opener closes 6-8 inches, stops, and reverses. Both sensors are showing steady LED so apparently they are not out of line or blocked by an object. I checked the door and it is balanced and does not bind when moved manually. The chain was sagging so I tightened it as specified in the users manual - no go. I'm at a loss on what to do next. Thanks for any help you can provide.

[WOOKIE]

Mine occasionally acts up like that and what I do to have it re -learn it is to hold the close button down until it fully closes then, push the button to open it and , it closes normally from then on until it acts up again. 

[jmm]

Thanks - that has worked for me in the past as well. Not this time though. The unit is only 8 years old. Any idea what the normal lifespan of these things is?

[WOOKIE]

No set time. I have replaced some 20 year's old and some as young as 5 years.  Too many factor's to set a life span.  They work until they cannot work anymore. You can blame planned obsolescence some things but with today's  junk on the market you can't even count on that. ex: dead out of the box -> I have experienced it many times. My last was a compressor  this month which tested good with a megger meter (Ω) but, pulled locked rotor amps.  

[jmm]

I think its in the control unit - the sensors are working, I checked all connection, the motor runs, the door isnt binding and seems balanced, and I cannot get it to travel more than 6-8 inches holding down the switch. The board is ~$100 and a new unit is $140. Easy choice. Thanks for your perspective.