Air conditioning keeps stopping

What it usually means

A split system that shuts itself down mid-run is rarely broken in the ordinary sense. It has decided that continuing would damage something, and the timing tells you what. Note how long it runs before it stops, whether it restarts on its own, and whether the indoor fan keeps going after the cooling stops — those three observations narrow this fault more than any amount of resetting.

The commonest protective stop is about ice. If air is not moving properly across the indoor coil, the coil gets colder than the design allows, and at some point the unit either stops to let it thaw or reports a freeze-up. That is why a dirty filter presents as a system that works beautifully for twenty minutes and then quits, recovers overnight, and does the same thing the next day. The same protective logic covers the outdoor unit in reverse: on a very hot day, or with a coil packed with leaves, it cannot reject heat fast enough, its pressures and discharge temperature climb, and it stops before the compressor is damaged. Both of these get worse in exactly the weather that makes them noticeable.

The second family is communication. An indoor unit and an outdoor unit are two appliances joined by a cable, and if the message between them fails, the system will not run — it usually shuts down promptly and reports a communication error. Rodent damage to interconnecting cable, water in a connection, a loose terminal after a hot summer, or work done nearby all cause it, and the useful diagnostic is that a communication fault stops the system in seconds rather than after a productive run. One brand attributes some undefined errors to electrical noise from other equipment, which is worth knowing because it explains the ones that appear only when something else in the building is running.

The third is not a fault at all. Timers, sleep modes and week schedules are easy to set accidentally on remotes with dozens of buttons, and a unit that goes off at ten each night and cannot be persuaded otherwise is following an instruction. Similarly, a unit that reaches its set point and stops is doing its job — the distinguishing question is whether the room is actually at the set temperature when it stops, because a system stopping at target and a system stopping short look identical from an armchair.

The last thing to check is the supply. An outdoor unit on its own circuit trips like any other appliance, and a breaker that goes after ten minutes of running is telling you about a load rather than a short. What not to do is reset repeatedly: each reset re-energises whatever the protection caught, and on a compressor the damage from repeated restarting is cumulative and expensive.