People describe it the same way nearly every time. A hard metallic snap, then one corner of the door drops four or five inches and jams there, the panel sitting diagonally across the opening. Often it was closed and nobody was near it. A loose end of cable lies coiled on the slab, the drum at that top corner has spun free, and the opener, if it was running, has stalled and started clicking. That is a lift cable, and on this coast it is one of the most common single failures on a garage door.
What the cable is doing up to that moment
There is one cable at each bottom corner. It hooks over a pin on the bottom bracket, runs up the inside of the vertical track and wraps into a grooved drum at the end of the spring shaft. The spring stores the energy, the drums turn, and the cables are the pair of steel ropes that translate that energy into lifting the door. Both sides work together, which is why losing one does not simply make the door heavier. It makes it lopsided, instantly, with the whole load transferred to the remaining side. The rollers on the low corner get dragged sideways in the channel and the top section takes a twist it was not built to take.
Why they rot where you cannot see it
A cable is not a rod. It is dozens of fine wires laid into strands and wound round a core, and that construction is what makes it both strong and flexible. It is also a network of narrow gaps that never dries out. Salt water wicks into those gaps and corrosion starts on the inner wires, working outward. So the outside can look gray and unremarkable while a good share of the cross section has gone. That is why the usual advice about watching for rust does not protect you here. Sprinkler overspray on the lower part of the cable and hose water off the driveway both feed it, and shaded alley garages behind the older bungalows dry out slowest of all.
Two places to look, with the door shut
The first is the bottom four inches, where the cable curves through the bottom bracket. Water runs down and stops in that bend, grit collects in it, and the curve is worked hard on every trip. The second is up at the top plate, where the cable beds into the drum groove. Damp sits between two steel faces there and it never sees sunlight. Signs on either: a broken wire standing proud of the lay, a length that has gone visibly thinner, a flattened section, a kink, or a single strand unwound and lifted away. Fine rust powder on the floor below a bottom corner counts too. Keep your fingers away from the loop over the pin while you look, and if one cable shows any of it, both are the same age.
The rag test
It costs nothing and it finds what your eyes miss. Take an old rag, hold it loosely around the accessible run of cable with the door closed, and draw it slowly along. A sound cable lets the cloth slide. A cable with a broken wire in it catches the fabric and pulls threads out, and you feel the snag before you can see the burr that caused it. Do it only with the door down, never with your hand wrapped around the cable, and never while anyone is standing at the wall button. If the rag snags anywhere along either side, stop using the door and book it. A cable that catches a rag today is a cable that parts within weeks, and its partner has done identical work.
Why nobody sensible does this one at home
The bottom bracket is the reason. With the door closed the spring is at full wind, so that bracket is under maximum pull, and taking its bolts out sends the load somewhere. Usually through a hand. The top end is no friendlier: setting a cable means winding the drum, holding the shaft, matching tension on both sides so the door lifts level, and locking drum set screws onto a clean shaft. Salt seizes those set screws routinely, and one that shears off turns a cable job into a shaft and drum job. Springs, cables and bottom brackets are the three items on a garage door that stay with the trade, and any fitter in San Diego County will tell you the same thing.
The cost, and what to do while you wait
Both cables replaced runs an estimated $135 to $245. Specify galvanised, which costs a little more per set and lasts materially longer inside a mile of the water. Since the drums have to come off anyway, ask for the drum grooves and the spring to be assessed at the same time, because a grooved drum quietly chews the new cable. Meanwhile: pull the release cord, keep the opener switched off at the wall if you can, and do not try to lift the door by hand, since one cable is now holding a load meant for two. If the door has jammed part way up and the garage is open to the street, mention that when you call from anywhere in Oceanside, because it changes the priority of the visit.