My AC is running constantly but the house won't cool below 80 — what's wrong?

Nine times out of ten in Tucson this is low refrigerant, a dirty condenser coil, or both. When a unit runs constantly and can't hit setpoint, the system is working at partial capacity — it's moving heat but not enough of it. First thing I check is the delta T (temperature split) across the evaporator coil. Should be 18-22°F. If it's less, you've got a refrigerant or airflow issue. Second, pull the disconnect on the condenser and look at the coil — in Tucson that desert dust packs in tight and kills efficiency fast. A clogged coil can drop your system 30-40% before you even notice. Third check is the capacitor. A weak capacitor lets the compressor limp along instead of running at full speed — common on units your age in our heat. None of these are expensive fixes if you catch them early. If refrigerant is low, you'll also want to know why — it doesn't just disappear. A leak check is worth it on a 9-year-old system before you dump more refrigerant in.