It won't turn on at all
Dead thermostat screen
Likely causes
- Dead thermostat batteries.
- A tripped breaker cutting power to the system.
- A safety float switch (full condensate pan) cutting the low-voltage circuit.
- A failed transformer or control board.
Safe checks you can do yourself
- Replace the thermostat batteries.
- Check the breaker and reset the AC breaker once if tripped.
- Take a flashlight look at the indoor unit's drain pan — if it's full of water, the float switch is doing its job.
When it's time to call
If fresh batteries don't wake it, the breaker re-trips, or the pan is full. A re-tripping breaker is a fault, not a nuisance — stop after one reset. A full pan means the drain line needs clearing before the system will run again.
Red flags — stop and call now: Breaker trips again immediately → leave it off and call.
Air conditioning repair & service
It runs, but blows warm air
Outdoor unit runs, air stays warm
Likely causes
- Low refrigerant from a leak.
- A dirt- or debris-blocked outdoor coil — a classic during SoCal heat waves; the coil can't dump heat.
- A compressor that runs but no longer pumps.
- Big duct leaks pulling in hot attic air.
Safe checks you can do yourself
- Replace a dirty filter.
- Clear leaves, dryer lint, and vegetation at least 2 feet around the outdoor unit — don't open anything, just clear around it.
- Feel two vents: if air is cool at one and warm at another, suspect ducts, not refrigerant.
When it's time to call
Refrigerant work is EPA-licensed-only territory — there is no safe DIY here. Honest note: “topping off” refrigerant every summer without fixing the leak is renting a repair; ask any company you call to find the leak, not just refill.
Air conditioning repair & service
Weak airflow from the vents
Weak airflow everywhere
Likely causes
- A clogged air filter.
- A coil beginning to ice over — which chokes airflow further.
- A failing blower motor or wrong blower speed.
Safe checks you can do yourself
- Pull the filter and hold it to a light — if you can't see light through it, replace it.
- Make sure all supply registers are open.
- Listen at the indoor unit: is the blower actually spinning up?
When it's time to call
If a fresh filter doesn't restore airflow within a few hours. Blower motors and iced coils are tech territory; running a starved system stresses the compressor.
Air conditioning repair & service
Ice on the unit or copper lines
Iced coil, airflow-starved
Likely causes
- A clogged filter or blocked returns starving the coil of warm air, so it freezes.
- Blower problems doing the same thing.
Safe checks you can do yourself
- Switch cooling OFF and the fan to ON — let it thaw for 2–4 hours (put towels by the indoor unit; melting ice makes water).
- Replace the filter.
- Open every register, then try cooling again.
When it's time to call
If it re-freezes after a thaw and a fresh filter. Repeated freezing with good airflow points at refrigerant, and running an iced system can slug and kill the compressor — the most expensive part in the machine.
Air conditioning repair & service
Water leaking around the indoor unit
Water at the indoor unit or ceiling stain
Likely causes
- A clogged condensate drain line — algae and dust; the #1 cause.
- A rusted or cracked drain pan.
- A failed condensate pump.
- An iced coil melting off.
Safe checks you can do yourself
- Flashlight look at the pan and the drain-line outlet (usually a small pipe ending outside) — is the outlet bone-dry while the pan is full? That's the clog.
- Put a towel or shallow pan under any active drip.
When it's time to call
Promptly if there's a ceiling stain — attic units drip into drywall and the damage bill outruns the plumbing bill fast. Line clearing is a quick visit.
Air conditioning repair & service
Strange noises (buzzing, screeching, banging)
Outdoor buzzing/humming
Likely causes
- A failing capacitor or contactor — buzzing that precedes a no-start.
- A fan obstruction.
- Loose hardware vibrating.
Safe checks you can do yourself
- Look — don't reach — through the top grille for obvious debris like a twig against the fan.
- Note whether the fan still spins up.
When it's time to call
Soon if the buzz is new or the fan hesitates on start. Catching a $30 capacitor before it strands the compressor on a 100° day is the cheapest AC repair there is.
Air conditioning repair & service
Runs constantly / can't keep up (heat wave)
Can't keep up in a heat wave
Likely causes
- Physics — most systems are designed to hold about 20°F below outdoor temperature, so 78° inside at 103° outside is on spec.
- A dirty coil or filter stealing capacity right when you need all of it.
- Duct leaks dumping cold air into the attic.
Safe checks you can do yourself
- Put in a fresh filter.
- Clear 2 feet around the outdoor unit.
- Close blinds on the sun side.
- Set a realistic target and let it run — short setbacks make it work harder.
When it's time to call
If it can't hold within ~5° of a reasonable setpoint, ices up, or this is new behavior at temperatures it used to handle. Honest read: a struggling-but-running AC in record heat is often normal; a change in behavior is the real signal.
Air conditioning repair & service