Nothing happens. You lower the thermostat and the house stays quiet. No blower, no hum from outside, nothing.
A completely dead system is actually easier to diagnose than one running badly, because the list of things that cut everything off at once is short. Work through it in order and you will either fix it or know exactly what to tell a technician.
Start at the Thermostat
Confirm the display is on. A blank screen means the thermostat is not receiving power, which is a different problem from the system not responding.
If the display works, check mode is COOL, the setpoint is several degrees below room temperature, and the fan is set to AUTO. Then try setting the fan to ON by itself.
That last test is the most informative thing you can do in thirty seconds. If the indoor blower starts, the thermostat has power, the low voltage wiring is intact, and the indoor unit is energized. Your problem is in the cooling circuit rather than in the basics. If nothing happens even on FAN ON, the fault is upstream.
Check Both Breakers
Central air conditioning uses two circuits, and people commonly check only one.
The indoor air handler or furnace has its own breaker, usually 15 or 20 amps. The outdoor condenser has a separate, larger breaker, typically 30 to 60 amps, often a double pole.
Reset properly. Push the breaker firmly to OFF first, then back to ON. A tripped breaker frequently rests in a middle position that looks like ON but is not making contact.
If either breaker trips again immediately, stop there. Repeated tripping on an air conditioning circuit means a genuine fault, and each reset sends the compressor into another start attempt under fault conditions.
The Service Switch and the Disconnect
Two switches exist specifically to cut power for servicing, and both get left off.
Near the indoor unit there is usually a switch that looks exactly like an ordinary light switch, often in a closet, attic, or garage. It gets flipped by accident, particularly after anyone has worked in that space.
Outside, beside the condenser, there is a disconnect box. Some have a pull-out block, some a lever, some a breaker. If work was done recently, or if the box has been opened, confirm it is fully engaged.
Also check the furnace or air handler access panel. Many units have an interlock that cuts power when that panel is not properly seated, so a filter change with a badly refitted door kills the system.
The Condensate Float Switch
This one causes more confusion than any other, because the system appears broken while working exactly as designed.
A float switch shuts the system down when water backs up in the drain pan. If the condensate line is blocked, the system stops rather than letting water overflow into the ceiling.
Look for standing water in the pan under the indoor unit. If you find it, the drain line is the actual problem. Clearing it restores operation, and the approach is covered in our guide to a system that smells musty, which shares the same root cause.
Given how much condensate systems produce in this climate, this is worth checking early rather than late.
The Low Voltage Fuse
Most air handlers and furnaces carry a small blade fuse on the control board, typically three or five amps, protecting the 24 volt circuit.
When it blows, the thermostat goes dark and nothing responds, while line voltage to the equipment remains perfectly fine. It looks like a dead thermostat.
A blown low voltage fuse almost always means a short in the thermostat wiring or in something it feeds. Replacing the fuse without finding the short just blows the replacement. Common causes are a pinched wire, damaged cable in an attic, or a shorted float switch.
The Capacitor
If the indoor blower runs but the outdoor unit does nothing, or hums without the fan turning, the capacitor is the leading suspect.
Capacitors store and release the energy needed to start the compressor and fan motors. They degrade with heat and age, and heat is something this climate supplies in quantity. A weak capacitor produces exactly this symptom: an audible hum with no rotation.
Sometimes a failed capacitor is visibly bulged at the top or leaking. Often it looks perfectly normal and only testing reveals it.
This is not homeowner work. Capacitors hold a dangerous charge even after power is disconnected, and they must be discharged before handling.
The Contactor
The contactor is the relay that switches line voltage to the outdoor unit when the thermostat calls for cooling.
Its contacts pit and corrode from repeated arcing, and eventually they fail to close properly. The result is an outdoor unit that never energizes despite the thermostat calling correctly.
Insects are a genuine cause here. Ants in particular are drawn to the contactor and can jam it or create a short across the contacts, which is common enough along the Gulf Coast to be worth mentioning.
The Transformer
The transformer steps line voltage down to the 24 volts the control circuit uses. When it fails, everything low voltage dies at once, including the thermostat.
Symptoms are identical to a blown control board fuse, which is why testing rather than guessing matters. A transformer usually fails because of a sustained short somewhere downstream, so the cause needs finding either way.
Compressor Overload
Compressors have internal thermal protection that shuts them down when they overheat. When it trips, the outdoor unit goes quiet while the indoor blower keeps running.
The distinguishing pattern is intermittent. The system works, stops on a hot afternoon, then works again after cooling for a while. That cycle points at overheating, which itself points at a dirty coil, low refrigerant, or a failing compressor.
A compressor tripping on overload repeatedly is a warning. Each event is hard on the windings, and the failure that follows is the most expensive one in the system.
After a Storm or an Outage
This deserves its own section here, because it is a seasonal pattern rather than an occasional one.
When power is restored after an outage, it rarely comes back cleanly. Utilities restore in stages, and the process produces voltage transients and brief brownouts as load is picked up. Equipment that was fine before the outage can be dead afterwards.
What typically fails is the control board, the capacitor, or the transformer, because those are the components least tolerant of voltage irregularity. Contactors weld shut occasionally, which leaves the outdoor unit running continuously rather than not running at all.
There is also a simpler possibility worth ruling out first. Some thermostats lose their configuration during an outage and return to factory defaults, which may not include your mode or schedule. Others hold a lockout for a few minutes after power returns, to prevent the compressor restarting against system pressure that has not equalized. That delay is deliberate and usually lasts around five minutes.
Before assuming damage, wait ten minutes, check the thermostat settings, and check both breakers. If the system was running when the power dropped and will not start afterwards, and those checks come up clean, something in the control circuit probably took the hit.
This pattern is the main practical argument for surge protection at the panel, since HVAC equipment is usually the most expensive thing on the circuit and cannot be unplugged ahead of a storm.
What Not to Do
- Do not reset a breaker more than once. Twice is the limit, and the second time should be the last.
- Do not open the outdoor unit or the air handler electrical compartment. Capacitors remain charged.
- Do not bypass a float switch to get cooling back. It is protecting your ceiling.
- Do not add refrigerant, or have it added, without the leak being located.
Keep a Record
If the system has failed before, write down what happened and what fixed it. Date, symptom, and the part replaced is enough.
Patterns are what a technician most wants and what homeowners most often cannot supply. A capacitor replaced twice in three summers points somewhere different than a capacitor replaced once in ten years, and the second failure looks identical to the first without that history.
It also protects you on warranty. Parts carry their own coverage periods, and proving a part was replaced eighteen months ago requires the paperwork.
When to Call
Call if a breaker trips again after one reset, if the outdoor unit hums without the fan turning, if the thermostat is dead with the breaker on and fresh batteries fitted, if you smell burning, or if the system starts and stops within a few minutes repeatedly.
In peak summer, a dead system is worth treating as urgent rather than waiting, particularly in a household with elderly residents, young children, or anyone with a health condition affected by heat.
Final Thoughts
Roughly half of completely dead air conditioners come down to a tripped breaker, a flipped service switch, an unlatched panel, or a float switch doing its job. All four take minutes to check and none require tools.
What the FAN ON test buys you is direction. If the blower runs, you know the low voltage side is healthy and the problem lies in the cooling circuit, which narrows the list considerably.
One Way AC & Heating provides AC repair across Terrebonne, Lafourche and St. Mary parishes, including Houma, Raceland, and Morgan City. Free estimates, with pricing agreed before work starts.