Something smells wrong when the AC runs.

Smells are useful information because different ones mean genuinely different things. Here are the three that come up, in order of how urgently they matter.

Rotten eggs: leave the house

Natural gas is odourless. Utilities add a sulphur compound specifically so a leak smells like rotten eggs or sulphur, because that is a smell nobody ignores.

If you smell it, leave the house. Do not flip switches, do not use the phone indoors, do not go hunting for the source. Call the gas company or 911 from outside or from a neighbour’s.

This is not an air conditioning problem, and it is not a thing to book a service call for. It comes up here because plenty of Phoenix homes have a gas furnace sharing the same ducts and the same cabinet, so people reasonably associate the smell with the HVAC system.

The rare non-gas version is a dead animal in a duct or near the air handler, which smells similar enough that it is not worth gambling on. Treat it as gas first.

Burning: shut it off and look

Dust burn-off. The first time the system runs after months idle, dust on warm components burns off. Faint, brief, clears within minutes. This one is normal.

Hot electrical. A sharp smell like hot plastic or scorched insulation is a different thing entirely. Overheating windings in a blower motor or fan motor, a contactor arcing, or a connection cooking itself. Shut the system off at the thermostat and the breaker.

Mechanical. A belt or a seized bearing in older equipment smells like hot rubber or hot metal.

Anything in the second or third category is a diagnostic, and it is not a wait-and-see situation.

Musty: the coil and the drain

The common one, and the least alarming.

The evaporator coil runs wet all summer by design, because it condenses moisture out of the air. A coil and pan that stay damp grow biological film, and the first rush of air after the system cycles on pushes that smell through the house.

A plugged or slow drain line makes it much worse, because then there is standing water sitting in the pan. The two problems travel together.

What helps: clearing and flushing the drain, treating it through the season, cleaning the coil, and making sure the filter is not so restrictive that the coil runs colder and wetter than it should.

What does not help: putting something fragranced in front of a return grille, which covers the smell and leaves the water.

A note on vinegar and bleach

Vinegar down the condensate access tee every couple of months through cooling season is the cheap habit that prevents most of this. Bleach is harder on the pan metal and on the glue in the line, and in an attic it is hard on everything it drips onto.