Most AC issues come from dead thermostat batteries, a dirty filter, or a tripped breaker — the six-step sequence below covers each fix.
When your air conditioner stops blowing cold air, the fix is often simpler than you’d expect. This air conditioning troubleshooting guide walks through the six-step diagnostic sequence that covers the most common causes — dead thermostat batteries, clogged filters, and tripped breakers — before you call a pro. Most residential AC problems trace to these three sources, and catching them yourself saves both money and a long wait in the heat.
Why Your AC Stopped — The Three Likeliest Culprits
Before diving into the full sequence, these three checks resolve the majority of no-cool calls.
Dead thermostat batteries. A blank display doesn’t mean the system is broken — it means the batteries are dead. Replace them, confirm the thermostat is set to COOL and the temperature is below room temp, and set the fan to AUTO rather than ON.
A clogged air filter. A filter that blocks light when held up to the sun restricts airflow badly enough to freeze the evaporator coil or trigger a safety shutdown. Turn the system off, swap the filter, and restart. Replace filters every 1–3 months — monthly if you have pets or allergies.
A tripped circuit breaker. Locate your electrical panel and find the AC breaker. If it’s in the middle or OFF position, flip it firmly to ON. If it trips again immediately, don’t keep resetting it — that signals a deeper issue that needs a licensed electrician or HVAC tech.
Air Conditioning Diagnostics In Six Steps
If the three quick checks above didn’t solve it, work through this diagnostic sequence in order. Trane’s official troubleshooting documentation confirms this progression as the standard approach for US residential systems.
| Step | What To Check | What To Do |
|---|---|---|
| 1 | Thermostat operation | Confirm COOL mode, temp below room, fan on AUTO. Replace dead batteries. Wipe the faceplate clean — dirt can skew the temperature sensor. |
| 2 | Air filter condition | Turn off HVAC. Hold filter to light — if no light passes, replace it. |
| 3 | Circuit breaker and disconnects | Check the breaker panel. Also check the outdoor disconnect box (a small gray box near the unit) — confirm it’s in the ON position. |
| 4 | Outdoor condensing unit | Clear leaves and debris from around the unit. Confirm the fan is spinning and the compressor is running. Clean dirty condenser coils with a coil cleaner. |
| 5 | Ice on coils or lines | Ice means an airflow or refrigerant problem. Turn the system off and let it thaw completely. Check the drain pan and line for clogs — clear with a water-and-vinegar mix. |
| 6 | System reset | Turn off the thermostat, kill the breaker for 5 minutes, then restore power. Some units have a dedicated RESET button on the outdoor unit — press it if present. |
After each step, run the system and check whether cooling has returned before moving to the next step. Most issues catch at step 1, 2, or 3.
When Should You Call An HVAC Professional?
Several jobs are off-limits for DIY troubleshooting because they require specialized tools, certification, or licensed professionals.
Refrigerant checks and top-offs. Only a licensed HVAC technician can legally handle refrigerant. Uneven cooling on the evaporator coil or ice that returns after you’ve cleared it and replaced the filter suggests a leak that needs professional diagnosis.
Capacitor and contactor replacement. These components store dangerous voltage even after the unit is powered off. Use a multimeter with an MFD setting to test, but leave the replacement to someone trained.
Cracked drain pipes or hardwiring repairs. Any plastic or electrical component that requires cutting, soldering, or replacing a section of drain line is a pro’s job.
Burning smells or loud mechanical noises. These signal motor or compressor failure and should trigger an immediate service call.
For homeowners who prefer to handle basic maintenance themselves, having the right gear on hand makes the first few steps faster and safer. Check out our tested roundup of the best air conditioning tools for maintenance.
FAQs
Why is my AC running but not cooling the house?
The most likely cause is a dirty air filter restricting airflow, followed by frozen evaporator coils or a thermostat set incorrectly. Check the filter first — it causes this symptom more often than any other single issue. If the filter is clean and the thermostat is correct, look for ice on the indoor coils or outdoor lines.
How often should I change my AC filter?
Standard 1-inch filters should be replaced every 1 to 3 months. Homes with pets, allergies, or high dust levels should replace monthly. Thicker 4- or 5-inch filters can last 6 to 12 months but should still be checked every 90 days for visible dirt buildup.
Why does my AC breaker keep tripping?
A breaker that trips repeatedly after being reset signals an electrical overload — often a failing compressor, a shorted wire, or a bad capacitor. Do not keep resetting it. Call a licensed HVAC technician to diagnose and repair the underlying issue before it causes further damage to the system.
References & Sources
- Trane. “Air Conditioner Troubleshooting.” Official diagnostic sequence for residential AC systems.