First hot week hits and the service guys get swamped with no-cooling calls. Thing is, plenty of those turn out to be a setting, a dirty filter, or a breaker that tripped. Here's how I run through it, from the easy stuff to the harder stuff.
1. Thermostat settings
Check that it's on Cool, the setpoint's below room temp, and the fan's set to Auto. If the fan's on On, it'll blow room-temperature air between cycles. Swap the batteries too if it's got them.
2. The air filter
A clogged filter chokes airflow. It's the number one cause of weak cooling. If it's gray, toss it. Two minutes, done.
3. Both breakers
Central system's got two: one for the indoor furnace or air handler, one for the outdoor unit. Indoor fan runs but outside's dead quiet? Check the outdoor breaker and the disconnect box by the condenser.
4. Is the outdoor unit running?
Head outside. Fan should be spinning, unit should hum steady. If it hums but the fan won't turn, capacitor's a likely bet. Shut the system off till it's fixed. Running a compressor that can't start will wreck it.
5. A dirty outdoor coil
Cottonwood fluff, grass clippings, dryer lint, they all mat onto the coil and trap heat. Kill the power, then rinse the coil gentle from the inside out with a hose. Keep 2 feet clear around the unit.
6. Ice on the lines or coil
Frost on the copper line outside, or ice on the indoor coil, means airflow's too low or the refrigerant charge is off. Flip the system to Off and the fan to On to thaw it. Takes hours. Swap the filter. If it ices up again, call a pro.
7. The condensate float switch
Whole system shut down and there's water in the drain pan? Float switch did what it's supposed to. Clear the drain and it'll usually fire right back up.
8. Measure the temperature split
Let it run 15 minutes, then measure air at a return grille and a supply register near the air handler. Healthy system'll show about a 15 to 20°F drop. Way less than that, with a clean filter and coil, points to refrigerant or compressor trouble.
Still calling a tech? Tell them what you checked, including the temperature split. Cuts diagnosis time, and on hourly billing that's cash in your pocket.