The majority of air conditioner faults come down to restricted airflow, not a failing compressor. A dirty filter, blocked coils, or a frozen evaporator explain most of the calls people make thinking their unit has died. All three are checkable in fifteen minutes and free to fix — the genuinely expensive fault, a refrigerant leak, is comparatively rare and always needs a certified technician.
How an air conditioner actually works
An air conditioner moves heat out of the house rather than creating cold, and that distinction matters for diagnosis. Refrigerant absorbs heat from indoor air at the evaporator coil, carries it outside, and releases it at the condenser coil before returning to repeat the cycle. Two fans support this: one blows indoor air across the cold evaporator coil, the other blows outdoor air across the hot condenser coil to help it shed heat.
Restrict either airflow path and the whole system suffers — less heat gets removed indoors, the evaporator coil runs colder than intended until it ices over, and the compressor works harder for less benefit. This is why a $15 filter causes more service calls than almost any actual mechanical failure.
Find your fault by symptom
| Symptom | Most likely cause | Full guide |
|---|---|---|
| Runs but blows warm air | Dirty filter, blocked coils, or low refrigerant | AC not blowing cold guide |
| Ice visible on the indoor coil or pipes | Restricted airflow forcing the coil below freezing | AC freezing up |
| Water pooling around the indoor unit | Blocked condensate drain | AC leaking water |
| Turns on and off every few minutes | Oversized unit, dirty filter, or a thermostat fault | AC short cycling |
| Blank thermostat or no response | Dead batteries, tripped breaker, or a wiring fault | Thermostat not working |
| Musty or sour smell from the vents | Damp coil growing mould, or a blocked drain pan | Related problems below |
| Loud rattling or buzzing from outside | Debris in the outdoor unit, or a failing capacitor | Related problems below |
| Outdoor unit silent, indoor fan runs | Tripped breaker, or a failed outdoor component | See circuit breaker guide |
Why airflow explains almost everything
Restricted airflow does not just reduce cooling — it actively creates new faults through a predictable chain:
- Less heat is removed from indoor air
Reduced airflow across the evaporator means less warm indoor air makes contact with the cold coil, so less heat is absorbed and carried away.
- The evaporator coil runs colder than designed
With less warm air passing over it to balance the refrigerant’s cooling effect, the coil temperature drops further than intended.
- Moisture on the coil freezes
Once the coil surface drops below freezing, condensation on it turns to ice rather than draining away as designed.
- Ice restricts airflow further, accelerating the whole cycle
The growing ice layer blocks even more airflow, so the problem compounds until the system is barely functioning — at which point people usually notice.
Hold a disposable filter up to the light. If you cannot see through it clearly, replace it. Check monthly during heavy use, and rinse or replace reusable filters on the same schedule. This single habit prevents a large share of the faults on this page before they start.
The one thing that is never DIY
An air conditioner does not consume refrigerant during normal operation. If a system is low, refrigerant has leaked out somewhere, and simply adding more without finding and repairing that leak means paying to do it again a few months later. Handling refrigerants requires certification in most countries, and venting them is illegal in many places. Any work involving the sealed refrigerant circuit belongs with a certified technician, full stop.
Getting ready for the season
A few minutes before the season’s first heavy use catches most faults before they become emergencies on the hottest day of the year:
- Replace or clean the filter.
- Clear at least 60 cm (2 ft) of vegetation and debris around the outdoor unit.
- Rinse outdoor condenser fins gently with a hose — never a pressure washer.
- Check the condensate drain runs freely.
- Run the system for a short test cycle before you actually need it, so a fault shows up as an inconvenience rather than an emergency.
When to call an HVAC technician
Call a technician for anything involving the refrigerant circuit, for a compressor that hums but will not start, for a breaker that trips specifically when the outdoor unit starts, for a coil that freezes repeatedly even with a clean filter, or for electrical faults inside the outdoor or indoor unit’s control compartment.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
