My garage door goes down all the way but then automatically opens back up all the way

Hi. I have seen this before, although usually not from chamberlain garage door openers.

Pull the release cord that unlocks the door from the opener, and manually open the door ALL the way open.

Then lock back in place in the open position.

Then use automatic opener to shut door.

It will close very slowly, but should close all the way.

Then, open the door again using the remote, and the door will once again open slowly, this time it should open all the way.

Then close door with remote, and door should now close as it is supposed, with the proper speed.

The slow opening and shutting is the system position sensor resetting itself.

This is why your door is acting up.

I would also look up by the belt drive connects to the front of the garage door opening, to make sure there isn't any dirt, cobwebs, or other obstructions on the sensor that may cause it to re-close the way it does.

Don't tighten the chain. It sounds like you have either a broken door spring or the drive gear in the motor unit is going bad. Release the opener from the door and operate it by hand. It should be easy enough to raise that your wife can do it with no problems and it should stay put when released about waist to shoulder high.

If the door hits the garage floor and then reverses to the open position you need to adjust the 'down limit' which is on the side of the motor unit. Turn this control in the opposite direction of the arrow printed on the cover. Some tweaking will probably be necessary.

Okay here we go.

Make sure you put the black cup back on the end of the motor shaft and that it is pressed all the way on. Make sure the RPM sensor is plugged in. If neither of these is the issue, then you probably did not have the motor shaft pushed all the way back when you tightened the coupler after replacing the worm gear. Loosen that coupler up and from the front of the motor (opposite end in which the coupler is on) push the shaft back as far as it will go. Hold it there while you tighten the coupler back up and then give it a try.

There is a sensitivity switch for both directions, open and close. If you access the garage door opener in the ceiling, you will see two adjustment screws for calibrating these. I would highly recommend following the instructions in your manual, as these are life safety adjustments! Your sensitivity for closing the door is adjusted to the positive. You just need to back it off a little. Place a 2x4 on the floor of the garage where the door comes down. and when the door hits the 2x4, it should automatically come back up. If not, you adjusted it too far. Hope this helps. and remember, check the manual for your specific adjustment procedures.

align sensors again even though both kights are on.

IF IT HITS THE FLOOR AND GOES BACK UP. YOU NEED TO ADJUST THE DOWN LIMIT.

IF IT GOES BACK UP BEFORE HITTING THE FLOOR YOU NEED TO ADJUST THE SENSITIVITY OR THE SAFETY EYES.

TRY THE DOOR MANUALLY AND MAKE SURE IT WORKS SMOOTHLY.

The door obstacle detector(reversing switch) needs adjustment, or the down limit sensor needs adjusting, or one of the lift springs is loose or broken. First, remove power from the opener, pull the red door release rope, and try to lift the door up manually. You should be able to lift it all the way open with litle effort. If it opens ok, pull the door down about halfway and let go. If it is properly balanced, it should stay where you left it. If the door fails either one of these tests, it needs spring replacement or adjustment. If the door moves ok, refer to your owners manual on how to adjust the reversing switch, linit switches, and photocell sensors at the door bottom.

Okay...upon further inspection, my left spring has broken in two. Going to call around for some quotes to replace the spring. Thanks for the suggestions.

I can't believe it! My opener works fine a 10 degrees and above but below 10 degrees it would go down a little ways stop and go back with the light flashing. I checked the electric eye and found it would go out of alignment when the temp drop. Who knew!
I made the adjustment and it now works fine.