The door was working fine for 8 years, last week it went down to about 2 inches from the floor, then went back up. No obstructions. I the started to adjust the chain...bad idea. Now in is inconsistant in it's operation-sometimes it goes down all the way, sometimes it goes all the way down, then comes back up.

Remove negative cable from battery, connect a test light to negative terminal, opposite end off light to negative battery post. ( let sit for 3 to 5 minutes). Then start pulling fuses, when the light goes out that's your bad circuit. God-Bless! Its worth a try.

broken springs...door to heavy for motor to lift call a pro

Deb, that reversal is a child safety device - if the bottom edge of the door detects contact with any object, it'll immediately reverse so it doesn't kill a child. Or a cat, dog, whatever.

The opener mechanism should have a stop switch that prevents the door from closing hard against the floor - that stop switch needs to be adjusted so that the door will stop JUST before hard contact. That'll take care of it "bouncing" off the floor & reversing. As to it reversing before it touches the floor... examine both guide rails closely to see if there's any obstruction that the door's bottom edge may be contacting to trigger the child protection reversal switch.

remove the door panel, to see whats is the problem

couple things it could be something as simple as a pencil down the defroster blocking the blend door that seperates the condensor coils from the heater core it lets in varing amounts of air hot or cold depending temp setting. blend door is probably clicking if fan is running.make a hook out of a short piece of wire,coat hanger ,but only 8 or 10 inches long work it around through the floor vent opening . not a fish hook just a small circle bent 2 a 45 degree angle if u can bump the obstruction while switching hot 2 cold and floor 2 defrost it should work down and fall out the floor vent

Check alignment of the obstruction sensors .... they shoot a light beam across the doorway about 6 inches above the floor. One is mounted on each side of the door track near the floor.

Try this link
http://www.garagedooropeners.us/helpgifs/chamberlain_troubleshooting.html

I would try jumping a wire across the white & red terminal to see if your safety sensors are working Properly! If your door opens you have a loose connection or faulty sensors.

pull the door seal back and look behind it. if you see 1/4 inch screws every 6-8 inches then you can re-straighten the door.

• take the door off of the refrigerator and remove the handles
• lay it down on a flat surface, like a large table or hard floor (not carpet)
• loosen (dont take out) all of those screws behind the door seal. this should relax the metal part of the door so you can twist it and make it flat against the floor. 
• once it is completely flat on the floor on all corners then tighten all the screws, starting with the center of each side and work towards the corners.
• when you reinstall the door make sure you line up the door correctly before tightening the hinge bolts.

what do you mean by rack. do you mean fears. do you have right sprigs on door, is door balanced. lift half way and let go (Disconnected by arm). It should stay put or go slowly up or down. If not, springs are bad, I put a 1/4 hp on 18' wood door. lasted 27 years so far. H/p not the problem.