It's 100°F outside and your AC is running — but your house isn't getting cool. Before you call for a $150 service call, check these five parts in order. Four of the five are under $50 and take less than 30 minutes to replace.
1. Run Capacitor ($15-$25) — The #1 Summer AC Failure
The run capacitor stores energy to keep the compressor and condenser fan motor running. When it fails — and it's the single most common AC failure — the compressor hums but won't start, or the outdoor fan spins slowly or not at all. You might hear a clicking sound from the outdoor unit.
How to check: Look at the capacitor through the access panel. If it's swollen, bulging, or leaking oil, it's dead. Even if it looks fine, a capacitor can fail without visible signs — a $15 multimeter test is definitive.
The fix: This is the easiest AC repair you can do yourself. Turn off power, discharge the old capacitor, note the MFD and voltage ratings, and swap it. Shop OEM capacitors →
2. Contactor ($25-$40) — The AC's Main Switch
The contactor is an electrically-operated switch that sends 240V power to the compressor and fan motor when the thermostat calls for cooling. When the contactor fails, the outdoor unit gets the signal but can't turn on, or it chatters and buzzes without fully engaging.
How to check: With the AC calling for cooling, look at the contactor through the access panel. If the plunger is pulled in but the unit isn't running, the contacts are burned. If the plunger isn't pulling in, the coil may be dead.
The fix: Turn off power, note the amperage rating (20A, 30A, or 40A), and swap the contactor. Shop OEM contactors →
3. Condenser Fan Motor ($85-$160) — Pulls Air Through the Coil
The condenser fan motor pulls outdoor air through the condenser coil to release heat. When it fails, the compressor overheats and shuts down on the high-pressure safety switch. You'll notice the outdoor unit is running but the fan isn't spinning, or the fan spins slowly.
How to check: If the fan isn't spinning when the AC is running, try spinning the blade by hand (with power off). If it spins freely, the motor has failed. If it's stiff, the bearings are seized.
The fix: This is a moderate DIY job — disconnect the old motor, note the HP, RPM, and rotation direction, and swap. Shop OEM condenser fan motors →
4. Air Filter (Free to Check) — Don't Skip This
A clogged air filter restricts airflow across the evaporator coil, causing it to freeze. The result: your AC runs but blows barely-cool or room-temperature air. This is the most overlooked cause of poor cooling and costs nothing to check.
How to check: Pull the filter and hold it up to light. If you can't see through it, it's clogged. Replace it immediately. Check it monthly during summer.
5. Low Refrigerant — This One Needs a Tech
If you've checked the capacitor, contactor, fan motor, and filter and the system still isn't cooling, you likely have a refrigerant leak. Signs include: ice forming on the refrigerant line at the outdoor unit, the larger copper pipe (suction line) is warm instead of cold, or the system short-cycles.
This requires a licensed technician with EPA certification to diagnose, locate the leak, repair it, and recharge the system. However, you can save money by ordering OEM parts yourself — technicians often mark up parts 100-200%.
Bottom Line
Check parts in this order: capacitor → contactor → fan motor → filter → refrigerant. The first three are the most common AC failures and all are available with same-day shipping from National HVAC Parts.
Need help identifying the right part? Enter your equipment model number on any category page, or use our free diagnostic tool.