Your AC is running, the fan is blowing, but the air coming out of the vents is warm — or at least not cold enough for a Phoenix summer. This is one of the most common summer service calls in the Valley, and it has a fairly short list of causes. Here’s how to diagnose it systematically.
Most Likely Causes — Ranked by Frequency in Phoenix
| Cause | What You’ll Notice | DIY Fix? |
|---|---|---|
| Dirty condenser coil | Gradual warming, higher bills, unit runs constantly | Rinse with hose — pro for deep clean |
| Low refrigerant (leak) | Ice on lineset, hissing sound, gradual warm-up | No — EPA cert required |
| Clogged air filter | Weak airflow, warm air, possible ice on evap coil | Yes — replace filter immediately |
| Failed compressor | Fan runs, no cold air, loud startup noise | No — major repair |
| Thermostat set wrong / failed | Fan runs but compressor doesn’t engage | Check settings first |
| Duct leaks | Specific rooms are warm, others are fine | Partial DIY — tape accessible leaks |
Step-by-Step Diagnosis
Step 1: Check the thermostat. Make sure it’s set to COOL (not FAN only) and the setpoint is at least 3–4°F below current indoor temperature. A thermostat in FAN mode blows unconditioned air — warm in summer.
Step 2: Check the air filter. Pull it out and hold it to light. If it’s uniformly gray, replace it immediately, then wait 30 minutes for the system to recover. A severely clogged filter starves the evaporator of airflow, causing it to ice over. The ice then blocks all airflow. If the evaporator is iced, turn the system to FAN ONLY for 1–2 hours to thaw, then run on COOL with a fresh filter.
Step 3: Go outside and look at the condenser. Is the fan spinning? Can you feel warm air coming from the top of the unit? If the fan is running but discharge air isn’t warm, the compressor may not be engaging. If the unit is off entirely, check the disconnect box next to the condenser and the circuit breaker inside.
Step 4: Look at the refrigerant lines (the two copper pipes going into the condenser). If you see ice or frost, you have either low refrigerant or an airflow problem. This requires a technician.
For Phoenix AC repair, call Discount AC & Refrigeration at 480 478-2616. We’re available 6 AM–midnight, 7 days a week — including July 4th when your AC decides to quit. We also serve Gilbert, Scottsdale, and surrounding cities.
Frequently Asked Questions
Answered by Discount AC & Refrigeration — EPA Certified · ROC #361623 · Phoenix, AZ
