AC problems • Troubleshooting guide
AC Won’t Turn On? Common Causes and What to Do
Nothing happens when you lower the thermostat, or the indoor blower runs but the outdoor unit stays silent. Start here.
Or dial (213) 214-0220 — available 24/7.
The short answer
When an AC will not start at all, the fault is usually in the chain that tells it to run: thermostat, power, safety switches, then the control board and the parts that start the compressor. Work through that chain from the easy end. A thermostat with dead batteries or a blank screen cannot call for cooling. A breaker that tripped during a power surge leaves the whole system dead. A full condensate pan can trip a float switch that shuts the system off on purpose, to prevent a leak.
If the thermostat is alive, the breaker is on and the indoor blower runs but the outdoor unit does nothing, the problem is outside: a capacitor, contactor or the outdoor disconnect. Those are quick for a technician to confirm and are not something to poke at yourself. One important rule: reset a tripped breaker once. If it trips again, stop and call.
Most common causes
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Thermostat has no power or is set wrong
Dead batteries, a blank display, a schedule that switched to Off, or a set point above the room temperature all mean the system is never asked to start.
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Tripped breaker or blown fuse
A surge, a brownout or a failing part can trip the AC breaker at the panel or blow the fuses in the outdoor disconnect. Everything looks dead until it is reset by hand.
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Condensate float switch tripped
Many systems have a safety switch in the drain pan. When the drain clogs and water rises, the switch cuts the AC off to protect your ceiling. The fix is the drain, not the switch.
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Failed capacitor or contactor
The capacitor gives the compressor and fan their starting kick; the contactor is the relay that switches them on. When either fails the outdoor unit hums, clicks or stays silent.
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Furnace or air-handler door switch
If a filter change left the blower compartment door slightly open, the door safety switch keeps the whole system off. Closing it fully is the whole fix.
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Control board or wiring fault
A low-voltage wire chewed by a rodent, a loose thermostat wire or a failed control board breaks the signal between thermostat and equipment.
Safe checks before you call
None of these involve opening the equipment, touching wiring or bypassing a safety switch. If a check needs a tool or a panel removed, it is not on this list on purpose.
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Wake the thermostat
If the screen is blank, replace the batteries. Set it to Cool with a set point well below the room temperature and wait five minutes; many systems have a built-in start delay.
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Look at the breaker panel
Find the breaker labelled AC, condenser or air handler. If it sits in the middle position, switch it fully off and then on once. If it trips again, leave it off and call.
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Check the furnace or air-handler door
Make sure the blower compartment panel is seated and latched. Do not remove the panel; just confirm it is closed.
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Check for water in the drain pan area
Look for standing water under the indoor unit or a full pan without touching wiring. A tripped float switch usually means a clogged drain that needs clearing.
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Confirm the outdoor disconnect is not pulled
Near the outdoor unit there is a small box with a pull-out handle or switch. Confirm it is in the On position. Do not open the unit itself.
Stop and call if…
How an AC24Repair technician diagnoses this
The technician traces the call for cooling from the thermostat forward. They confirm the thermostat is sending a signal, check that low-voltage power is present at the equipment, and test the safety switches: door switch, float switch and any high-pressure cut-out. That alone finds a large share of no-start calls.
If the indoor side is fine, they move to the outdoor unit and test the capacitor, contactor and compressor windings with the proper meters, and check the fuses in the disconnect. Because a failed capacitor and a failed compressor can look alike from the outside, they never guess: the readings decide. You then get the diagnosis and the repair price before anything is replaced.
Repairs that may follow the diagnosis
Which one applies, and what it costs, depends on what the technician finds. You hear the price before any work starts.
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Thermostat replacement or rewiring
A failed thermostat or a broken low-voltage wire is repaired or replaced and the system is tested through a full cycle.
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Capacitor or contactor replacement
A routine part swap at the outdoor unit that gets the compressor and fan starting again.
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Condensate drain clearing and float switch reset
The drain line is cleared and flushed so the safety switch releases and stays off.
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Control board or fuse replacement
When the control board has failed or its fuse keeps blowing, the technician finds why before fitting a new one.
Clear pricing before the work starts.
Diagnostic visit fees depend on the time of day. We publish the confirmed diagnostic fees and explain the repair price before work begins.
Help with this across Los Angeles
Technicians come to homes in the San Fernando Valley, West, Central and Northeast Los Angeles.
AC won’t turn on: questions homeowners ask
Why does the thermostat work but the AC does nothing?
The thermostat only sends a low-voltage signal. If that signal is not reaching the equipment, or a safety switch is holding the system off, the thermostat can look normal while nothing runs. That gap is exactly what the diagnostic visit traces.
How many times can I reset the breaker?
Once. A breaker that trips a second time is doing its job. Leave it off and call.
The indoor fan runs but no cold air comes out. Is that the same problem?
Related but different: the indoor side is working and the outdoor unit is not. Common causes are a failed capacitor or contactor, or a tripped disconnect fuse. See our guide on an AC running but not cooling for the airflow side of that symptom.
Could it be the thermostat batteries?
Very often. A blank or dim screen is the giveaway. Replace them, set Cool, and wait five minutes for the start delay before deciding.
Do you charge more if it turns out to be something simple?
No. The diagnostic visit fee depends only on the time band of the visit. If the fix is small, you pay the visit fee and the small repair, and you hear both before work starts.
Tell us what’s happening. We’ll take it from there.
Call any time. We schedule a technician within 24 hours and explain the price before repair work starts.
Or dial (213) 214-0220 — available 24/7.