The air conditioner runs but the house will not cool

The first question is whether the outdoor unit is running at all. Stand next to the condenser while the thermostat is calling for cooling. You are listening for the compressor, a low steady hum, and watching for the fan on top to spin. What you find splits the problem in half.

Indoor air moving, outdoor unit silent

The blower is fine and the refrigeration side is not running. Check the breaker for the outdoor unit and the disconnect switch in the box beside it, which sometimes gets left open after other work. If the breaker has tripped, reset it once. If it trips again, stop, because a breaker that trips twice is reporting a fault rather than a nuisance.

Both running, air barely cool

• Check the filter. Restricted airflow drops capacity long before it stops the system.
• Look at the refrigerant line where it enters the house. A thick jacket of ice on the insulated line means the coil has frozen. Switch the system to fan only and let it thaw for a few hours before anything else.
• Check whether the outdoor coil is packed with cottonwood, grass clippings or leaves. It rejects heat through those fins and cannot do it through a mat of debris.

What ice actually means

A frozen coil is a symptom, never the cause. It comes from too little airflow across the coil or too little refrigerant in the system, and the second one means there is a leak, because refrigerant is not consumed. Topping up a system without finding the leak buys a season at most and vents the charge again. Ask for the leak to be found.