Your AC is humming, the fan is moving, but the air coming out is barely cool, or worse, room temperature. Before you spend $79 on a service call, walk through this list. Half the time it's something you can fix in 10 minutes.
Before you start: safety first
Two rules. One: turn the system OFF at the thermostat before doing anything outside the unit. Two: do not open any panel that requires tools, there are 240V circuits inside that will hurt you.
The 6-step diagnostic
1. Check the filter
This is the #1 cause of weak cooling, easily 60% of our "not cooling" calls. A filter that's gray or visibly clogged restricts airflow so badly the evaporator coil freezes solid. Symptoms: weak airflow, ice on the indoor unit's copper lines.
Fix: Replace the filter. If the coil is already iced, turn the system OFF, leave the fan ON for 4 hours to thaw, then restart with a clean filter.
2. Check the thermostat
Sounds dumb, but: is it set to "Cool" and not "Fan only"? Is the setpoint actually below room temperature? Is the battery dead (if it's battery-powered)?
Also: smart thermostats sometimes lose their schedule after a Wi-Fi outage. They'll show "Cool 72°" but actually be in "Hold" mode at 78°.
3. Check the outdoor unit
Walk outside. Is the big unit (condenser) running? You should hear a fan and a low compressor hum. Touch the side, it should be warm but not hot.
If it's not running: check the breaker panel, there's usually a dedicated double-pole breaker for the AC. Also check the disconnect box on the wall next to the unit (silver or gray box, sometimes pulled out as a service shutoff).
If only the fan is running but not the compressor: almost certainly a bad capacitor. ($249-449 for the part + service call.)
4. Check the outdoor coil
Look at the outdoor unit's fins (the radiator-like surfaces). Are they covered in cottonwood seeds, grass clippings, or pet hair? If yes, the coil can't reject heat and the system can't cool.
Fix: Turn the system OFF. Spray the outside of the fins with a garden hose from the inside out (top to bottom, gentle pressure). Don't bend the fins.
5. Check the indoor coil drain
Your indoor unit (in a closet, attic, or basement) has a drain pan and a PVC drain line. If it's clogged, the system shuts off cooling to prevent water damage but the fan keeps running.
Fix: Locate the drain line outlet (outside, usually a 3/4" PVC pipe near the foundation). Pour a cup of distilled vinegar in the access port near the unit (T-fitting with a screw cap on top of the line). If the line is fully blocked, you need a tech with a wet-vac.
6. Check vents and returns
Walk through the house. Are all the supply vents open? Are any return vents blocked by furniture, rugs, or curtains? A blocked return = restricted airflow = same symptoms as a dirty filter.
What if you've done all that and still no cool air?
Now it's almost certainly one of three things, none of them DIY:
| Issue | Symptoms | Typical fix |
|---|---|---|
| Low refrigerant (leak) | Warm air, hissing sound, ice on copper lines | Find leak + repair + recharge: $400-1,200+ |
| Failed compressor | Outdoor unit hums but no cool air, frequent breaker trips | $1,800-3,500 (often time to replace whole system) |
| Reversing valve stuck (heat pumps) | System blows hot when set to cool (or vice versa) | $450-800 |
The "do I need a new system?" question
If your tech tells you the system needs a major repair (compressor, evaporator coil, refrigerant leak in inaccessible piping), here's the math:
- System is <10 years old: Repair almost always wins. Modern systems are built for this.
- System is 10-14 years old: Compare repair cost to replacement. If repair is >40% of replacement, replace.
- System is 15+ years old: Replace, even if repair is "only" $800. You'll be back in two summers anyway, and you're paying R-22 prices for a system that's 30% less efficient than today's tech.
- System uses R-22 refrigerant: Phased out in 2020. R-22 now costs $130/lb wholesale and a typical recharge needs 4-8 lbs. Replace.
- Can I just add refrigerant myself?
- No, and it's illegal in most cases, refrigerant handling requires EPA 608 certification. Also, simply adding refrigerant doesn't fix the leak that caused the loss. Within weeks you'll be in the same spot.
- Why does my AC freeze up?
- Three causes in order of frequency: dirty filter, low refrigerant, failing blower. Always check the filter first.
- Should I cover my outdoor AC unit in winter?
- No, despite what the internet says. Modern condensers are designed for outdoor exposure year-round. Covers trap moisture and create homes for rodents that chew wiring.
- What's the lifespan of a residential AC in NJ?
- 12-18 years for a properly-maintained, properly-sized system. Northern NJ's climate is gentle on AC compared to the South, you have winter rest periods that let units recover.