A central air conditioner that does not work has failed at one of five stages in its operating sequence: the thermostat is not calling for cooling, the electrical power is not reaching the equipment, the outdoor condenser is not starting, the indoor blower is not moving air, or the refrigeration circuit is running but not producing cold air. By identifying which stage the system stops at, you narrow the causes from dozens of possibilities to two or three before you ever open a panel.
The fastest diagnostic sequence is to go to the thermostat, then walk outside to the condenser, then check the indoor air handler or furnace. At each location, answer one question: is it doing what it should be doing? The thermostat should display COOL ON or a similar indicator. The outdoor unit should have the condenser fan spinning and the compressor running with a low steady hum. The indoor unit should have the blower moving air through the registers. The air from the registers should be cold. Wherever the sequence stops — that is where the problem is.
Central AC Not Working: Quick Symptom Diagnosis
| Symptom | Most Likely Cause | DIY Fix? |
| Nothing happens at all — no fan, no compressor | Thermostat, tripped breaker, or furnace door switch | Yes — check settings and breakers |
| Indoor blower runs, outdoor unit silent | Capacitor, contactor, or tripped breaker at outdoor unit | Check breakers; capacitor needs a tech |
| Outdoor unit hums but fan and compressor don’t start | Failed capacitor | No — lethal shock risk, call technician |
| Outdoor fan spins, compressor not running | Compressor capacitor, contactor, or compressor failure | No — call technician |
| Compressor runs, air is weak and not cold | Dirty filter, frozen coil, or low refrigerant | Check filter; if frozen, thaw; if neither, call tech |
| Compressor runs, air is strong but not cold | Low refrigerant or failing compressor | No — call technician |
| System short cycles on and off | Dirty filter, frozen coil, oversized unit, or low refrigerant | Check filter first |
1. Thermostat: The Setting That Costs Nothing to Check
A central AC that does nothing — no indoor blower, no outdoor condenser, no sound at all — has either a thermostat that is not calling for cooling, or no power reaching the equipment. Before doing anything else, verify three settings on the thermostat. First, the mode must be set to COOL, not OFF or HEAT. Second, the set temperature must be below the displayed room temperature by at least 3°F to 5°F. If the thermostat is set to 75°F and the room is 76°F, the dead band may be preventing the call. Third, if the thermostat is battery-powered, replace the batteries. A thermostat with dead batteries cannot close the relay that sends the 24-volt cooling signal to the equipment.
If the thermostat display is blank, the thermostat has no power at all — see the dedicated article on Honeywell thermostat blank screens for the full diagnostic sequence. If the display is on and the settings are correct but the equipment does not respond, remove the thermostat from the wall plate and use a short piece of wire to briefly jump the R (power) and Y (cooling) terminals. If the outdoor condenser starts, the thermostat is faulty and must be replaced ($25 to $80). If the condenser does not start, the problem is in the equipment, not the thermostat.
2. Tripped Circuit Breaker or Disconnect Switch
Central air conditioners have two sources of electrical power, and either one being off prevents the system from running. The outdoor condenser is typically on a dedicated 240-volt circuit in the main electrical panel. The indoor air handler or furnace is on a separate 120-volt circuit. If the condenser breaker has tripped, the outdoor unit is dead. If the air handler breaker has tripped, the indoor blower is dead. Check both breakers. Reset a tripped breaker by flipping it fully to OFF, then fully back to ON. If the breaker trips again immediately, there is a short circuit in the equipment — call a technician. Do not keep resetting a breaker that trips repeatedly.
The outdoor condenser also has a disconnect box — a small metal box mounted on the wall near the unit — that contains either a pull-out handle, a lever, or fuses. Verify that the disconnect is in the ON position. A disconnect that was turned off during the winter for maintenance and never turned back on in the spring is a zero-cost fix that requires only flipping a switch or pushing a handle back in.
3. Failed Capacitor: The Outdoor Unit Hums but Won’t Start
A central AC condenser that hums loudly but the fan does not spin and the compressor does not start has a failed capacitor. The capacitor is a cylindrical component inside the outdoor unit’s electrical compartment that provides the electrical boost needed to start the compressor and the condenser fan motor. When the capacitor fails, the compressor draws locked-rotor current — a loud 60 Hz hum — for a few seconds until its thermal overload switch cuts power. The unit sits silent for several minutes while the overload cools, then tries again.
The capacitor is the most common component failure in a central AC system. It is a wear item with a typical service life of 5 to 10 years, and it tends to fail on the hottest day of the year when the system is under maximum electrical load. Capacitor replacement costs $150 to $300 and takes a technician 20 minutes. Do not attempt to replace a capacitor yourself. Capacitors store a lethal electrical charge even with the power disconnected, and the terminals must be discharged with an insulated screwdriver before handling. A capacitor shock can stop a heart.
4. Failed Contactor: The Click That Never Comes
The contactor is a heavy-duty relay inside the outdoor unit that switches the 240-volt power to the compressor and the condenser fan. When the thermostat calls for cooling, the 24-volt signal from the thermostat energizes the contactor’s coil, the contactor closes with an audible click, and power flows to the compressor and fan. A contactor that has pitted or burned contacts, an open coil, or insect debris between the contacts will not close, and the outdoor unit will not start.
A failed contactor produces a distinctive symptom: the thermostat calls for cooling, the indoor blower runs, but the outdoor unit is completely silent — no hum, no click, no fan, nothing. The 24-volt signal is reaching the contactor but the contactor is not responding. A visual inspection of the contactor (turn off power at the disconnect first) reveals burned, pitted, or ant-infested contacts. Ants are attracted to the electromagnetic field of the contactor coil and frequently nest inside contactors, blocking the contacts from closing. Contactor replacement costs $150 to $300.
5. Dirty Air Filter or Frozen Evaporator Coil
A central AC that runs — the outdoor condenser is humming, the indoor blower is moving air — but the airflow is weak and the air is not cold has either a dirty air filter restricting airflow across the evaporator coil, or the coil itself is frozen. Replace the air filter if it has been more than 30 days since the last change. Open the air handler or furnace panel and inspect the evaporator coil. If the coil is covered in frost or ice, turn the thermostat to OFF and the fan to ON. Let the blower run continuously for 1 to 4 hours to thaw the ice. Place towels under the air handler to catch meltwater.
After the coil has thawed and the filter is clean, restart the AC. If the coil freezes again within days, the problem is low refrigerant or a failing blower motor. A system with a refrigerant leak will gradually lose cooling capacity over weeks or months before the coil begins freezing regularly. A failing blower motor will produce unusual sounds — humming, clicking, or squealing — before the airflow drops enough to freeze the coil.
6. Low Refrigerant or Refrigerant Leak
A central AC that runs — the compressor is humming, the blower is moving air at full strength — but the air is not cold has a refrigerant problem. The compressor is running, but the refrigerant is not absorbing heat from the indoor air because the charge is too low to complete the evaporation cycle properly. The air from the registers feels cool but not cold, or it feels like a fan blowing room-temperature air.
Additional signs of low refrigerant: the larger copper pipe at the outdoor unit (the suction line, wrapped in black foam) is not cold and sweaty during operation, ice forms on the evaporator coil or the suction line, a hissing or bubbling sound comes from the indoor or outdoor unit, and the system runs continuously without satisfying the thermostat. Air conditioners are sealed systems. Refrigerant does not get used up. If the charge is low, there is a leak. An EPA-certified technician must locate the leak, repair it, evacuate the system, and recharge it to the precise weight specified on the nameplate. Leak repair and recharge costs $500 to $1,500.
7. Clogged Condensate Drain Tripping the Float Switch
Central air conditioners produce condensate that drains through a PVC pipe to a floor drain or a condensate pump. A float switch in the drain pan or the condensate pump reservoir cuts the 24-volt power to the thermostat’s Y (cooling) terminal when the water level rises. The thermostat displays COOL ON, the indoor blower may run, but the outdoor condenser never starts because the 24-volt cooling signal is interrupted at the float switch.
This is one of the most common causes of a central AC that “was working yesterday and suddenly does not work today.” The condensate drain clogged gradually, the float switch finally tripped, and the AC shut down. Clear the drain line with a wet-dry vacuum from the outside end of the pipe. Pour a cup of warm water with a few drops of bleach into the drain pan to kill algae. If the system uses a condensate pump, verify the pump is receiving power and the float moves freely. Empty the pump reservoir manually if it is full.
8. Indoor Blower Motor Failure
A central AC where the outdoor condenser is running — the fan is spinning, the compressor is humming, the suction line is cold — but no air is coming from the registers has a failed indoor blower motor. The system is producing cold air at the evaporator coil, but the blower is not distributing it through the ductwork. The coil will freeze solid within 15 to 30 minutes because there is no airflow to keep it above freezing, and the compressor is at risk of liquid slugging.
Turn the AC off immediately if the outdoor unit is running but no air is coming from the registers. A blower motor that has failed completely must be replaced: $400 to $800 for a standard PSC motor, $800 to $1,500 for an ECM variable-speed motor. A blower motor that hums but does not spin may have a failed start capacitor ($150 to $300), which is a separate repair from the motor itself. Check the air filter and the blower compartment before calling a technician. A loose wire connection at the blower motor or the control board can also cause a no-blower condition.
FAQ: Common Questions About Central Air Not Working
Can I reset my central AC by turning it off and on?
Turn the thermostat to OFF. Turn the circuit breaker for the outdoor condenser to OFF. Wait 5 minutes. Turn the breaker back ON. Turn the thermostat to COOL and set the temperature below room temperature. This power-cycle sequence resets the contactor, the capacitor (if it still has some residual capacitance), and any control board logic that may have locked out after a fault. If the system starts and runs normally after a power-cycle reset, the fault was transient. If it does not start, the fault is persistent and requires a technician.
One zone of my central AC is cold but the other is not. Is that a central AC problem?
No. Uneven cooling between rooms or zones is a duct balancing or zoning damper problem, not a central AC problem. The AC produces the same cold air regardless of where it goes. If some rooms are cold and others are warm, the duct dampers for the warm rooms may be closed, the ducts serving those rooms may be disconnected or leaking, or the zoning system’s motorized dampers may have failed. This is a duct or zoning repair ($200 to $800), not an AC repair.
Follow the Operating Sequence from Thermostat to Compressor
A central AC not working has stopped at one of five points: the thermostat, the electrical supply, the outdoor condenser start circuit, the indoor blower, or the refrigeration circuit. Walk the sequence. The thermostat should call. The breakers should be on. The outdoor unit should start. The blower should move air. The air should be cold. Wherever the sequence stops, that is where the problem lives.
Check the thermostat and the breakers yourself — those are zero-cost, zero-risk checks. Replace a dirty air filter. Clear a clogged condensate drain. If those checks do not restore cooling, the problem is the capacitor, the contactor, the blower motor, or the refrigerant charge, and each of those requires a technician. The diagnostic visit costs $100 to $200. The repair costs $150 to $1,500 depending on what failed. The capacitor is the most common failure and the cheapest fix. The compressor is the least common failure and the most expensive. The odds are on the capacitor.
