What causes an air conditioner to stop working?
When an air conditioner stops working, the most common culprits are power or control problems, airflow issues, or refrigerant-related faults. Quick checks—like ensuring the circuit breaker isn’t tripped, replacing a dirty filter, and confirming the thermostat is set correctly—often resolve simple failures.
Electrical and power problems
Electrical issues are a frequent reason a cooling system won’t start or will shut down unexpectedly. These problems are usually observable and fixable at the property level, but some require professional attention to avoid safety hazards.
Before the following list, consider these common electrical fault indicators and steps you can take.
- Tripped circuit breaker or blown fuse protecting the outdoor condenser or air handler.
- Outdoor disconnect switch is off or faulty, interrupting power to the unit.
- Wiring issues or loose electrical connections inside the unit or at the thermostat.
- Faulty electrical components such as capacitors or relays (start/removal devices) that prevent the compressor or fans from starting.
Addressing electrical problems usually starts with a safety-first check of the main breaker, disconnects, and visible wiring. If a reset or replacement is not straightforward, a licensed technician should perform the work to avoid shock or fire risk.
Airflow and filtration problems
Even a powerful cooling system can underperform or shut down if airflow is restricted or filtration is poor. Clean airflow is essential for cooling efficiency and component protection.
These are the typical airflow and filtration issues to review.
- Clogged or very dirty air filters restricting air entering the evaporator coil.
- Blocked return air ducts or closed/blocked supply vents reducing airflow to the unit.
- Cracked or leaking ductwork that wastes conditioned air and lowers efficiency.
- Frozen evaporator coil due to restricted airflow or very low refrigerant pressure.
Maintaining clean filters, clearing obstructions, and ensuring adequate ductwork is crucial for reliable operation. If the coil frequently freezes, professional inspection is recommended to diagnose airflow or refrigerant issues.
Refrigerant and cooling cycle problems
The refrigerant cycle is at the heart of cooling performance. Problems here can cause the system to run without delivering adequate cooling or to shut itself off in safety modes.
Consider these refrigerant cycle issues when diagnosing a non-working AC.
- Low refrigerant charge due to a leak, resulting in inadequate cooling and potential system shutdown.
- Refrigerant leaks from coils, connections, or service ports, which require professional repair and recharging.
- Faulty metering device or expansion valve that misregulates refrigerant flow.
- Compressor or related pressure switches failing to maintain proper high/low-side pressures, triggering protection mechanisms.
Note: Handling refrigerants and diagnosing leaks should be done by licensed technicians in accordance with environmental and safety regulations. Leaks not only reduce cooling efficiency but also pose environmental and health risks.
Mechanical components and wear
Over time, mechanical wear and failed components can bring cooling to a halt. These parts are critical for the physical movement and operation of the system.
The following mechanical factors commonly cause outages or poor performance.
- Faulty capacitor that prevents the compressor or fan from starting or running poorly.
- Failed or pitted contactor in the outdoor unit that interrupts power to the compressor or fan.
- Bad or worn fan motor or bearing failure leading to reduced airflow or motor burnout.
- Significant compressor problems, including overheating or internal failures, that halt operation.
- Drain-related safety shutoffs (condensate overflow or float switch issues) triggering a shutdown to prevent water damage.
Mechanical failures often require skilled diagnostics and component replacement. If unusual noises occur or the unit trips frequently, a technician should inspect the system to prevent further damage.
Thermostat and controls
Controls govern when and how the system runs. A misconfigured or faulty control system can make it seem like the AC has stopped, even when components are functioning.
Key thermostat and controls issues to check include:
- Incorrect thermostat settings (cooling setpoint too high, fan on Auto vs. On, or mode set to Heat/Cool mis-match).
- Dead or weak batteries in programmable or smart thermostats affecting power and communication with the HVAC system.
- Loose, damaged, or corroded thermostat wiring causing intermittent signals.
- Software or firmware glitches in smart thermostats that prevent proper operation or communication with the outdoor unit.
Thermostat problems are often fixable by reconfiguring settings, replacing batteries, or rebooting the device. Persistent issues may require a professional to verify wiring and control board integrity.
Outdoor unit environment and installation factors
The outdoor condensing unit relies on clear space, proper airflow, and appropriate installation. Environmental and site factors can impede performance or trigger protective shutdowns.
Consider how the surroundings and installation affect operation.
- Debris, leaves, dirt, or vegetation blocking the condenser coils and reducing heat rejection.
- Inadequate clearance around the unit leading to poor airflow and overheating.
- Extreme outdoor temperatures or power supply fluctuations stressing the system.
- Poor installation practices or mismatched components for the home’s cooling load.
Improving outdoor airflow, trimming nearby vegetation, and ensuring an appropriate service clearances can restore performance. A professional check is advised if the unit is struggling in hot conditions or shows signs of overheating.
What to do next
When troubleshooting, start with simple, safe steps you can perform without specialized tools. If these steps don’t resolve the issue or involve refrigerants or electrical components beyond basic resets, contact a licensed HVAC technician for a thorough inspection and repair.
Recommended steps to take before calling a professional:
- Check the main circuit breaker and any outdoor disconnect to ensure power is supplied to the unit.
- Inspect and replace dirty air filters; ensure return and supply vents are open and unobstructed.
- Verify thermostat settings and battery status; perform a simple system reset if applicable.
- Observe any error codes or warning lights on the thermostat or indoor air handler and note them for the technician.
Having a technician confirm refrigerant levels, electrical integrity, and refrigerant leaks is essential for safety and compliance with environmental regulations.
Summary
Air conditioners stop working for a mix of electrical, airflow, refrigerant, mechanical, and control-related reasons. Start with quick checks—power, filters, and thermostat settings—then investigate airflow and outdoor conditions. If simple steps don’t restore function, or if refrigerant or electrical components appear involved, seek a licensed HVAC professional to diagnose and repair the system safely and effectively. Regular maintenance can reduce the frequency of these failures and extend the life of the unit.
