Your AC is running, the fan is blowing, but the room is not getting cool. This is one of the more frustrating AC problems because the unit appears to be working but is not doing its job. Fortunately, most causes follow a predictable pattern and can be worked through in order from the simplest to check to the most complex.
Quick answer: Start with the two most common and easiest checks: confirm the unit is set to cool mode (not fan), and clean the filter. These resolve a surprisingly large proportion of not-cooling complaints. If both are fine, work through the causes below in order before calling a technician.
Diagnostic Approach: Work From Simple to Complex
| Check first | Likely cause if wrong | Fix | DIY? |
|---|---|---|---|
| Mode on remote | Set to fan instead of cool | Switch to cool mode | Yes |
| Filter condition | Clogged filter, reduced airflow | Clean filter | Yes |
| Set temperature | Set too close to room temperature | Lower set temperature | Yes |
| Outdoor unit | Blocked or dirty condenser | Clear and rinse outdoor unit | Yes |
| Coil / pipes | Ice formation on coil | Switch off, thaw, clean filter | Yes (thaw only) |
| Room vs unit size | Unit undersized for room | Check tonnage; consider upgrade | Yes (check) |
| Refrigerant lines | Refrigerant leak | Call technician | No |
| Compressor / electrical | Compressor or electrical fault | Call technician | No |
Cause 1: Wrong Operating Mode
It sounds obvious but it is genuinely common, particularly after the unit was used in fan or dry mode. Check the remote display: the unit must be in cool mode (often shown as a snowflake icon) for the compressor to engage. If the remote shows fan mode, the unit will blow air but not cool it. Switch to cool mode and set the temperature at least 4 degrees below the current room temperature.
Cause 2: Dirty or Clogged Filter
A heavily clogged filter reduces airflow over the evaporator coil so severely that the unit can run for hours without meaningfully cooling the room. This is the most common cause of a running-but-not-cooling complaint and takes 15 minutes to check and fix. Clean the filter and run the unit again for 20 minutes. For the mechanism behind this, see why dirty filters reduce cooling efficiency.
Cause 3: Set Temperature Too High
If the set temperature is only 2 to 3 degrees below the current room temperature, the compressor will reach its target quickly and cycle off, leaving the room only slightly cooler than before. Lower the set temperature to at least 24 degrees and ensure it is noticeably below the current room temperature.
Cause 4: Outdoor Unit Blocked or Dirty
The outdoor unit releases the heat extracted from the room into the outdoor air. If the condenser fins are clogged with dust and debris, or if plants or walls are blocking airflow around the unit, heat rejection becomes inefficient. The refrigerant stays warmer than it should, reducing the system's ability to cool the indoor air. Clear any obstructions, maintain 30 cm clearance on all sides, and rinse the fins with a low-pressure hose.
Cause 5: Frozen Evaporator Coil
A frozen coil acts as insulation, blocking heat transfer between the room air and the refrigerant. The unit runs but delivers no useful cooling. You may see ice on the copper pipes coming from the indoor unit, or water dripping. Switch the unit off and let it thaw fully (1 to 3 hours), then clean the filter and test again. If it refreezes, the cause is likely low refrigerant and needs a technician. Full guidance is in why is my AC freezing up.
Cause 6: Unit Undersized for the Room
An AC that is one size class too small will run continuously at full load and still fail to cool the room on the hottest days. If the unit cools adequately on mild days but cannot keep up when it is above 38 to 40 degrees, undersizing is a likely factor. Check with the AC Tonnage Calculator. For the consequences of undersizing, see what happens when you use an undersized AC.
Cause 7: Refrigerant Leak
Refrigerant is the working fluid that carries heat from inside to outside. A leak means less refrigerant to do this work, reducing cooling capacity progressively. Signs include the unit running continuously without cooling, ice on the indoor pipes, and sometimes a faint hissing or chemical smell. Refrigerant is not consumed like fuel; a low level always means a leak somewhere. This needs a licensed technician to find the leak, repair it, and recharge the system.
Cause 8: Electrical or Compressor Fault
If the fan runs but there is no cooling and no ice formation, the compressor may not be engaging. This can be caused by a failed run capacitor (a relatively inexpensive component), a compressor relay fault, or a failing compressor. A technician can diagnose this with a multimeter and pressure gauges. A failed capacitor is a common and affordable repair; a failed compressor is the most expensive repair on the list, sometimes costing more than a new unit.
Check whether your AC is the right size for your room as a first step.
AC Tonnage CalculatorKey takeaways
- Check mode and filter first: these resolve the majority of running-but-not-cooling complaints.
- A frozen coil means switch off, thaw, and clean the filter before running again.
- If the unit cools on mild days but not on very hot ones, it may be marginally undersized.
- Refrigerant leak and compressor or electrical faults need a licensed technician.
- Work through the diagnostic table in order before calling for service.
Frequently Asked Questions
Why is my AC running but not cooling?
Start with the two easiest checks: confirm the unit is in cool mode (not fan), and clean the filter. These resolve most cases. If both are fine, check the outdoor unit for blockages, look for ice on the indoor pipes, and consider whether the unit is sized correctly for the room.
Why does my AC not cool enough on very hot days?
On days above 40 to 42 degrees, a marginally sized or underperforming unit may fail to reach the set temperature. Clean the filter, ensure the outdoor unit is unobstructed, and raise the set temperature slightly. If the problem persists in all conditions, the unit may be undersized.
How do I know if my AC has a refrigerant leak?
Signs include continuous running without cooling, ice forming on the indoor pipes, a hissing or bubbling sound, and sometimes a faint sweet or chemical smell. A refrigerant leak requires a licensed technician to locate, repair, and recharge.
Should I reset my AC if it is not cooling?
Switching the unit off at the circuit breaker for 30 seconds and back on can clear minor electronic faults and is a reasonable first step. If the problem persists after a reset, filter clean, and mode check, a technician is needed.
Sources and Further Reading
- Bureau of Energy Efficiency, India, AC troubleshooting guidance (beeindia.gov.in)
- ENERGY STAR, room air conditioner troubleshooting (energystar.gov)
- U.S. Department of Energy, common air conditioning problems (energy.gov)
General troubleshooting guidance. Refrigerant leaks and compressor or electrical faults must be diagnosed and repaired by a licensed HVAC technician.