AC troubleshooting by symptom: what to check before you call

A meaningful share of service calls end with a filter, a breaker or a thermostat setting, and the visit is billed all the same. Working through a short list first costs nothing, sometimes solves the problem outright, and when it does not, it gives the technician a much better starting point than "it stopped working".

Symptom 1: nothing happens at all

No air from the vents, no sound from the outdoor unit, nothing.

1. Thermostat. Blank display means dead batteries or no power to it. Replace the batteries before assuming anything else. Confirm it is set to COOL and the setpoint is below the room temperature.
2. Breakers. There are usually two, one for the air handler and one for the outdoor unit. A tripped breaker often does not look obviously tripped; switch it fully off, then on. Once only.
3. The disconnect. A box on the wall beside the outdoor unit with a pull-out handle. It gets switched off during landscaping or painting and not switched back.
4. The float switch. If the drain is blocked and the pan is full, the switch has cut the system off deliberately. Look for water at the air handler or in the emergency pan.
5. The air handler's own switch. Many installations have a light-switch-style disconnect near the unit, frequently mistaken for a light switch and turned off.

Stop here if: the breaker trips a second time. That means a fault drawing more current than the circuit permits, and repeated resets push faulted wiring past its rating.

Symptom 2: the fan runs but the air is not cold

Air comes out of the vents and it is room temperature or close to it.

1. Check whether the outdoor unit is running. Indoor fan running with the outdoor unit silent is the classic pattern, and it points at the contactor, the capacitor, the outdoor breaker or the control circuit.
2. Check the filter. If you cannot see light through it, replace it and allow an hour.
3. Check the thermostat fan setting. On ON rather than AUTO, the blower runs continuously and delivers uncooled air between cycles. This alone generates a great many calls.
4. Look for ice. On the large insulated line at the outdoor unit or on the indoor coil. Ice means the system must be thawed before anything can be measured.

Stop here if: the outdoor unit hums but the fan does not turn. Do not attempt to start the fan by hand. The capacitor holds a charge capable of injuring you after the power is off.

Symptom 3: water where water should not be

Wet floor by the air handler, a stain on the ceiling below an attic unit, or the system shutting itself off intermittently.

1. Switch the system off. It stops producing condensate, which stops the leak at source.
2. Find the emergency pan under an attic air handler. Water in it means the primary drain is blocked.
3. Check the drain outlet outside. Nothing coming out on a humid day while the system runs confirms a blockage.
4. Clear it if you can reach it. A wet/dry vacuum on the outdoor end of the drain line will often pull the blockage through.

Stop here if: water has reached drywall or ceiling material. That is a building problem with its own clock, and drying matters as much as the repair.

Symptom 4: a new noise

The sound tells you a good deal, so identify it before calling.

Stop here if: it is grinding or screeching. Running a motor with failed bearings destroys it, and the replacement is more expensive than the visit would have been.

Symptom 5: a smell

Burning or electrical. Power off at the breaker and call. This is the one category that does not wait.

Musty, strongest in the first thirty seconds. Biological growth on the indoor coil or in the drain pan. Unpleasant rather than urgent, and resolved by cleaning.

Dusty, on the first heating cycle of the year. Dust burning off heat strips after ten months idle. Normal, and it should fade within fifteen minutes.

Rotten eggs. Leave the house and call the gas utility from outside. Natural gas is uncommon in most homes here, but the smell is a mercaptan additive and it is never something to investigate yourself.

Symptom 6: it cycles wrongly

Short cycling means starting and stopping every few minutes. Causes range from an oversized system through a dirty filter, low charge, a thermostat in direct sunlight or too close to a supply register, to a control fault. What it has in common is cost: compressors draw their highest current at startup, and frequent starts wear the machine faster than long runs.

Constant running that never satisfies the thermostat is the opposite pattern with a similar list of possible causes, and it also happens legitimately on the hottest afternoons of the year, when a correctly sized system may simply be at capacity.

What you can check: filter, thermostat location, and whether the setpoint is realistic. Everything else needs measurement, because the same symptom sits on top of six different faults.

The five-minute check worth doing twice a month

Most of the symptoms above announce themselves gradually. A short routine catches them while they are still cheap, and it takes less time than reading about them.

1. Hold a hand at a supply register and at the return grille. The difference should be obvious. If supply air feels only slightly cooler than return air, something has changed.
2. Look at the filter. Monthly through cooling season, whatever the packaging says about ninety days.
3. Walk to the outdoor unit while the system runs. The fan should be turning steadily and the air leaving the top should be warm. Listen for anything new.
4. Check the drain outlet for water on a humid day.
5. Read a hygrometer. 45 to 55 percent indoors. A creeping humidity reading is often the first sign of a problem, well before temperature is affected.

Write down what you find with a date, even briefly. The value is not in any single reading; it is in noticing that the supply air stopped feeling as cold as it did in May, which is exactly the observation that separates a repair in June from a failure in August.

What is definitively not a homeowner job

• Anything involving refrigerant. Handling it requires EPA Section 608 certification. Venting it is illegal, and the sealed system is not serviceable without proper equipment.
• Capacitors. They store energy after the power is off and discharge through whatever completes the circuit.
• Any work inside the electrical panel or in the outdoor unit's cabinet beyond visual inspection with power off.
• Bypassing a safety. Float switches and high-limit switches exist because of specific failure modes. Defeating one converts an inconvenience into damage.

When the list above runs out, what you have collected is worth passing on: which parts run and which do not, what changed and when, what you already checked. That description is what turns an hour of diagnosis into twenty minutes of it. Whether you call Roman's Service AC repair or another local firm, leading with those observations rather than with a conclusion produces a better visit.

Frequently asked questions

How long should I wait after resetting the breaker?

Five minutes before expecting the compressor to restart. Many systems have a built-in delay that prevents short cycling, and it is doing its job rather than failing.

Can I pour bleach down the drain line?

Better not. Bleach corrodes metal components in the pan and the pump. Distilled vinegar or a proper condensate treatment works without the damage.

The system works after I hit the breaker. Is it fixed?

No. Something caused it to stop, and a reset that restores operation temporarily usually means a component is failing intermittently. It will recur, and typically on a hotter day.

Should I turn the system off if it is running but not cooling well?

Yes, if there is ice anywhere, and yes if there is any water where it should not be. Otherwise it can generally keep running until the visit, though a system struggling on low charge is being damaged the whole time it runs.

Current as of September 2026.