The garage door has a job that has nothing to do with opening and closing. When something gets under it, it has to stop and go back up. Most doors do this with two small sensors near the floor, one on each side, that talk to each other across the opening. If the beam is blocked, the door should reverse. If it does not, the door is trying to close on whatever is in the way. Dad may not have noticed. You may notice first when the door keeps coming down with the dog in its path or the cat on the threshold.
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Check the sensors first
Look at each sensor and find its small light. Both lights should be steady when the door is closed and the path is clear. If one is blinking or off, you have found the first problem. Wipe the lenses with a dry cloth. Dust and cobwebs on the lens are a common reason a sensor reports a problem that is not there, or stops seeing what it should. Check that nothing is leaning against the brackets and that no wire is cut, pinched, or pulled loose from the staples along the wall.
Align the sensors
The two sensors should face each other straight across the opening, mounted at the same height above the floor, usually a few inches up in the spot your manual shows. If one is tilted, loosen its bracket screw just enough to move it, aim it at the other sensor, and tighten it again without twisting the wire. Tilt is usually a bumped bracket, not a bad sensor. Watch the light on the receiver. It should go solid when the beam lines up. Move the bracket a little at a time and stop when the light holds steady.
Run the reverse test
Once the lights are steady, test the reverse. Lay a flat board, such as a piece of 2x4 lumber, on the floor in the center of the door's path. Close the door using the wall button or the remote. The door should go down until it touches the board, then stop and go back up. Repeat the test with the board placed near each side of the door. If the door keeps pushing on the board instead of reversing, stop pressing the button. That is the point to call a technician.
Test the manual release
Once a month, pull the emergency release cord. The door should lift by hand without binding, and it should reconnect when the opener runs again. If the door is hard to lift or stays locked, the release or the springs may need repair. Do not adjust the springs yourself. Springs under tension can cause serious injury, and that work belongs to a trained technician.
When to stop and call
Call a technician if a sensor will not stay aligned, if a light stays off after you have cleaned and tightened, if a wire looks damaged, if the door does not reverse on the board, or if the door has started making new noises or moving unevenly. Write down what you did and what you saw. A technician will want to know whether the lights were blinking, whether the door reversed on the first try, and whether the release cord worked. Those notes can save you a return visit.
Put the check on the list
Once you know the steps, the monthly check takes only a few minutes. Put it on the same list as the other household checks and make it one line: sensors lit, board test passed, release cord tested. Dad will roll his eyes at the list. He will also be glad the door is not trying to close on the dog.