From your description it sounds like when you came back to close the door after mowing something was blocking the safety beam or perhaps you bent the safety beam mount with the mower?? Anyway pulling the cord only disengages the door from the Genie screw and not from power etc. Stopping after 5 minutes is probably a heat sensor shutting the motor off. I would call the installer back and try to have a reasonable discussion with him. There probably isn't much wrong if anything. The manual is a good source of information but not very interesting reading.

Thanks Don, I should've mentioned the up force was at 9. I found and called the cust svc number 800-528-5236, and they made me lower the up force to 5, manually lift the door waist high and drop it. Since it slammed to the ground, their conclusion is that the door is not balanced and thus the GDO is doing all the work. they recommended a couple of repair guys to do what's called a Door Balancing. Does this make sense to you? Thanks again.

Ok, gotcha now. If I were their I would pull off the cover and see if anything inside looks worn. Check out the gear, if there is a bunch of white saw dust looking stuff the gear is wearing on you and in need of replacement. Also look for ground up metal on top of the opener itself right next to the sprocket and on top of the gear. If this is the case you will need a whole gear and sprocket assembly. If all that checks out good the problem is probably in the circuit in board in which I would invest in a new opener rather than trying to repair it. The board is going to cost around 90 bucks and you can buy a new opener for about 150 bucks with up to date technology. No, On the way up if the door becomes obstructed in some way it just stops. Good luck.

1/2" is acceptable but 3/4" is reasonably priced and will give you the most solid subfloor.