On the first hot week of summer, service companies get flooded with no-cooling calls. A good share of those turn out to be a setting, a filter or a tripped breaker. Here's the order I check things in, from easiest to hardest.
1. Thermostat settings
Make sure it's set to Cool, the setpoint is below room temperature, and the fan is on Auto. If the fan is set to On, it blows room-temperature air between cooling cycles. Replace the batteries if it has them.
2. The air filter
A clogged filter chokes airflow, and it's the most common cause of weak cooling. If it's gray, replace it. It takes two minutes.
3. Both breakers
A central system has two: one for the indoor furnace or air handler, and one for the outdoor unit. If the indoor fan runs but the outdoor unit is silent, check the outdoor breaker and the disconnect box next to the condenser.
4. Is the outdoor unit running?
Go outside. The fan should be spinning and the unit should hum steadily. If it hums but the fan doesn't turn, a capacitor is a likely culprit. Turn the system off until it's fixed. Running a compressor that can't start can damage it.
5. A dirty outdoor coil
Cottonwood fluff, grass clippings and dryer lint mat onto the coil and trap heat. With the power off, rinse the coil gently from the inside out with a hose. Keep 2 feet of clearance around the unit.
6. Ice on the lines or coil
Frost on the copper line at the outdoor unit, or ice on the indoor coil, means airflow is too low or refrigerant charge is off. Switch the system to Off and the fan to On to thaw it, which can take hours. Replace the filter. If it ices up again, call a pro.
7. The condensate float switch
If the whole system shut off and there's water in the drain pan, the float switch has done its job. Clear the drain and it will usually restart.
8. Measure the temperature split
Once it's been running 15 minutes, measure the air at a return grille and at a supply register near the air handler. A healthy system usually shows a drop of about 15 to 20°F. Much less than that, with a clean filter and coil, suggests a refrigerant or compressor issue.
Calling for service anyway? Tell the tech what you've already checked, including the temperature split. It saves diagnosis time, and on an hourly bill that's money.