A door that reverses before it reaches the floor usually means the safety eyes see a blocked beam. If it touches the floor and then pops back up, the travel limit or close force is usually off.
People assume the motor is dying. A lot of the time the motor is fine. The opener is reacting to something it treats as an obstacle. The useful split is where the reverse starts.
If it reverses before it touches the floor
Look low on both sides of the opening. Two small boxes face each other, maybe six inches off the concrete. One sends a beam. The other catches it. If that beam breaks, or the eyes think it broke, the door is not allowed to finish closing. That is on purpose. A kid, a dog, a bike wheel in the path. The reverse is the feature working.
What usually breaks the beam when nothing is in the way: dust on the lenses, a cobweb, a leaf, a box against the track, sun glare late in the afternoon, or a bracket bumped out of line. Road salt and grit cake on them over a winter.
Wipe both lenses with a soft cloth. Check that both LEDs are lit the way the sticker says when they are aligned. If one eye is dark or blinking, nudge the bracket until both settle. Do not force anything. Do not climb the track. More on that blink pattern lives on the sensor lights note.
After rain, wet leaves, a drifted bin, or a bumped bracket often start the same reverse. Wipe and clear the path before you touch travel screws.
If it kisses the floor then pops back up
Different story. The door reaches the slab, the bottom seal compresses a bit, then the opener reverses as if it hit a wall. Sensors can still be solid for this one. The down travel limit is often set a hair too far, so the motor keeps pushing after the seal already landed and treats the floor like an obstruction. A close force setting that is too light does the same dance when a sticky roller or stiff seal adds a little drag at the end.
Do not crank force to the max so it will smash through whatever is binding. That defeats the safety reverse. Tiny travel adjustments are model specific. Read the sticker on your unit before you twist anything. If you are not sure which control is which, stop and leave it for a shop.
This is also different from a door that stops a few inches short and hangs with daylight under the seal. That one never finishes the close. Here the door actually meets the floor, then climbs.
If the door binds or lists when you move it by hand
A sticky roller, bent track, or dragging weather strip can make the opener feel a hard stop and reverse the same way. The difference is whether the door fights you when you pull the emergency release and move it by hand.
If you pull the release and the door still scrapes or hangs on one side, leave it. That is track or roller work, not a lens wipe. Do not stand under a door that feels wrong. A roller hanging outside the rail needs a shop, not another close attempt.
What it is not
Not a door that opens by itself. Phantom open starts from shut. Close reverse starts mid close and climbs. Not a door that hangs stuck halfway. That one just sits. Here the door climbs after it tries to close. A silent remote leaves the opener quiet. Here the motor runs, then reverses. An opener that grinds like gravel chews drive parts in the head.
What not to do
Do not tape over the eyes so the door will smash shut. Do not spin travel or force controls in big jumps while the door moves. Do not stand under a panel that keeps reversing. If you need the car out, pull the release only when the door still feels even by hand, roll it slow from the side, then leave the opener alone until a shop checks eyes, travel, force, tracks, and seal.
I am not a contractor. This is just what shows up when a door refuses to finish a close and the opener is still humming along. A shop that does garage doors every day will check the eyes, travel, force, track, and bottom seal in one visit.
Safety notes on openers and reverse live on DASMA's garage door opener page.
The U.S. CPSC garage door operator page covers federal rules for automatic openers.
If the door will not stay down and you need someone local to look, get a quote.