One low corner usually means a cable stretched, frayed, or jumped its drum. Stop running the door and do not replace the cable yourself.
Look at the cables. Thin steel lines run from the bottom corners up to drums on the spring shaft. They keep both sides even when the spring turns. When one cable stretches, frays, or jumps a groove, that side loses its pull. The opener still tries. The door leans.
A frayed cable looks fuzzy, like a rope that has started to shed. Strands stick out. Sometimes the line sits slack off the drum. Sometimes it is still wrapped but shiny bare wire shows where it wore through against a hinge or the track. Do not poke it. Those strands can cut a finger, and a cable under load can whip if it finishes breaking.
If one corner hangs lower and a cable looks fuzzy, slack, or off the drum
That is the classic cable look. One side sits lower in the opening. One cable looks fuzzy, hangs loose, or has left its groove on the drum. The rollers may still sit inside the rails while the whole panel lists. Stop using the opener. Do not climb a ladder to poke the fuzzy line. Do not wrap tape around frayed strands and hope they hold for one more trip.
Pull the emergency release only if you need to free a car and the door still feels mostly balanced when you try it by hand, slow, standing to the side. If one corner fights you, the door twists, or a cable climbs farther off the drum, stop. Leave it. Use the house door.
If both cables look fine but the door still lists
Look again from the driveway without standing under the panel. If both lines still look wrapped and smooth, the lean may be a roller walking out of the track, a bent rail, or a tired spring that is not lifting both sides evenly. A door that screeches every open from dry or cracked rollers can also start to list before a cable fails in plain sight. If a wheel is sitting in open air, stop the opener. You are past the cable fork on this page.
If you already heard a bang or the door will not lift
That is spring territory. A broken torsion spring usually announces itself with one loud crack and a door that feels like a brick. The opener may hum. The door barely moves. When the spring snaps, a cable can go slack or take a shock it was never meant to take alone. Treat bang plus dead weight as spring work first. Do not wind cones.
What it is not
This is different from a spring bang that leaves the door dead heavy. A spring failure often stops the lift cold. A cable problem often lets the door move, just badly. Crooked. Scraping. One roller walking toward the edge of the track. Keep going and you can bend the track or drop a panel off the rollers.
It is also different from weeks of screech from dry rollers with the door still tracking even. Screech for months is often the warning. A fuzzy cable and a listing corner is the failure showing up in steel.
A door that hangs midway without an obvious fuzzy cable can still be cable, track, ice, or spring.
What not to do
Do not replace the cable yourself with a hardware store spool. The drums, the winding, and the spring tension all have to match or the next failure is worse. Do not stand under a listing door to “help” it. Do not mash the opener hoping the low side will catch up. Do not wrap the fuzzy strands in tape or zip ties and keep driving under it. Each wasted cycle can bend the track or finish a break while someone is in the opening.
Cold mornings make weak cables fail sooner. Salt and grit chew the bottom strands. None of that means a DIY spool is safe.
I am not a contractor. This is just what shows up when a door starts hanging uneven and people keep hitting the button hoping it will straighten out. A shop that does garage doors every day will replace the bad cable, check its pair, reseat the drums, and rebalance before they hand the opener back to you.
DASMA’s publications library hosts tech data sheets aimed at dealers and installers.
The U.S. CPSC garage door operator page covers federal entrapment rules for automatic openers. A listing door is also a crush hazard. Keep kids and pets clear of the opening until a shop has it secured.
If the door is listing and you want someone local to look, get a quote.