I have a Liftmaster 1200 series PN 41A4252-7C that will not operate from the door control or the remote. I get an approximately 3 second humming from the opener then a small light (not the opener light) on the back of the unit will flash 5 times, stop, then flash 5 times again etc. This happens whether the door is engaged with the trolley or released. Pressing the black button next to this light makes it stay lit, but activating the door control or remote gives the same result. Lifting the door manually is not difficult, so I think the spring is OK. The unit is mounted to an aluminum sectional door and was installed by the builder in 1996. Thanks for any advice you can offer.