AC running but not cooling usually comes down to one of five things: a clogged air filter, a frozen evaporator coil, low refrigerant, a failed capacitor, or dirty condenser coils. Start with the filter — it causes more than half of these calls and takes 30 seconds to check.
You get the call. Tenant says the AC's been running all night and the apartment's still 82 degrees. The unit is clearly on — fan's blowing, compressor's humming — but no cold air. Before you start pulling things apart, run through this flow. Every step eliminates the most common cause at that point.
The 5 Most Common Causes of AC Not Cooling
Dirty or clogged air filter
Restricts airflow across the evaporator coil, causing it to freeze or starve the system of heat exchange capacity. Most common cause. Check first, always.
Frozen evaporator coil
Ice on the indoor coil blocks airflow completely. Usually caused by a dirty filter, blocked return vents, or low refrigerant. Visual check on the air handler tells you immediately.
Low refrigerant (leak)
System doesn't have enough refrigerant to absorb heat. You'll see ice on the suction line, hear hissing or bubbling, and notice the system runs constantly without reaching setpoint.
Faulty start or run capacitor
Compressor hums but doesn't start, or the condenser fan runs erratically. Capacitor fails due to heat and age. Common on units over 5 years old in hot climates.
Dirty condenser coils (outdoor unit)
Can't reject heat. The system works harder, runs hotter, and loses cooling capacity. Check the outdoor coil — if it's caked with debris, the whole system is working against itself.
The Diagnostic Flow — Work Through These in Order
Don't skip ahead. Each step eliminates the cheap problems before you start looking at the expensive ones.
Check the Air Filter
Pull the filter and hold it up to the light. If you can't see through it, it's the problem. A filter clogged past its service life cuts airflow enough that the evaporator coil drops below freezing and ices over, blocking the remaining airflow entirely.
Replace it. If the coil is frozen (Step 2), you need to defrost first — but the filter check comes first because you need to know whether to fix the cause before you defrost.
Check for a Frozen Evaporator Coil
Open the air handler access panel. The evaporator coil should look dry and clean. If you see ice or heavy frost — on the coil itself or on the large copper suction line going into the air handler — the coil is frozen.
A frozen coil is a symptom, not a root cause. The root cause is either the dirty filter you just found, blocked return vents, or low refrigerant (Step 4). To clear it:
- Turn the system to Fan Only (heat and cool off) for 2–4 hours
- Do not run cooling while the coil is frozen — you will damage the compressor
- Once defrosted and the filter is replaced, restart and monitor for re-freeze
Check the Outdoor Unit (Condenser)
Go outside to the condenser unit. You're looking for three things:
- Is the fan running? Fan spins, compressor hums but doesn't cool — suspect capacitor (Step 4). Fan not running at all — check breaker and capacitor.
- Are the coils caked with debris? Dirt, cottonwood, grass clippings on the outside of the condenser fins cut heat rejection dramatically. A garden hose rinse from inside out (not outside in) can restore significant capacity.
- Is there ice on the suction line? The large insulated copper line going into the unit — ice here confirms the refrigerant is too cold, indicating low charge or restricted airflow.
Test the Capacitor
If the condenser fan is running but the compressor is humming and not starting (or starting and quickly shutting off on thermal overload), the capacitor is the first thing to check. Capacitors are the most common component failure on residential AC units.
To test:
- Shut off power at the disconnect box
- Wait 5 minutes for the capacitor to discharge (do not skip this — they hold lethal charge)
- Use a multimeter with a capacitance setting to test against the MFD rating printed on the capacitor
- Acceptable range is typically ±6% of rated MFD. Below that, replace it.
Capacitors are typically $15–40 in parts and a 15-minute swap. They're marked with dual ratings (e.g., 45/5 MFD) — the higher value is for the compressor, lower for the fan motor.
Check Refrigerant Charge (Requires Gauges & EPA 608)
If you've cleared the filter, the coil isn't frozen (or re-freezes after defrost), the capacitor is good, and the condenser coils are clean — you're likely looking at a refrigerant issue.
Signs that point to low refrigerant:
- System runs continuously without reaching setpoint
- Suction line ice or frost after defrost with clean filter
- Hissing or bubbling sound near the refrigerant lines
- Supply air temperature is only a few degrees below return air temp (healthy delta-T should be 14–22°F across the coil)
Refrigerant diagnosis requires manifold gauges. If you're not EPA 608 certified, this is where you hand off to a licensed tech. Adding refrigerant without finding the leak is a temporary fix — the refrigerant will leak out again.
When You Need to Call a Licensed HVAC Tech
Stop and call in if:
- The coil re-freezes after defrost with a clean filter and clear return vents
- You hear hissing, gurgling, or bubbling from refrigerant lines
- The compressor won't start even with a new capacitor
- You measure supply air temp within 5°F of return air temp (no delta-T)
- The unit is tripping a breaker repeatedly
Everything above that point — filter, frozen coil, capacitor, dirty condenser — is maintenance-level work. Refrigerant handling, compressor diagnosis, and electrical troubleshooting beyond the capacitor is licensed HVAC territory.
This is the same diagnostic flow FixAtlas walks you through — on every call.
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