AC Not Cooling? Troubleshooting Checklist Before Calling a Pro

It sounds obvious, but bumped switches, kids, and power outages reset thermostats constantly. In FAN ONLY mode, the blower runs but the compressor doesn't — you get airflow but no cooling. In HEAT mode, the system may not respond at all when outdoor temps are above the heat setpoint.

When the fan is set to ON, the blower runs continuously — even between cooling cycles. During off-cycles, the blower pushes unconditioned air through the ducts, making the air feel warm even though the system is working. Set it to AUTO so the blower only runs when the compressor is actively cooling.

This forces a cooling call. You should hear the outdoor compressor start within 1–3 minutes. If the outdoor unit doesn't start but the indoor blower runs, the problem is on the outdoor side (compressor, capacitor, contactor, or power). If nothing happens at all, the thermostat may not be sending a signal.

Battery-powered thermostats (most wireless models) lose function when batteries die — even if the display still shows faintly. Replace with fresh AA or AAA batteries. Hardwired thermostats with blank displays indicate a blown fuse on the furnace/air handler control board — that's a separate issue (see electrical section below).

Hold the filter up to a light. If you can't see light through it, it's restricting airflow. A severely clogged filter can reduce airflow 50%+ and cause the evaporator coil to ice over within hours. Replace the filter immediately — or temporarily run without a filter for a day to see if cooling improves (confirming the filter was the problem). Cost: $5–$30 for a replacement filter.

Closed vents in unused rooms, furniture pushed over floor vents, and rugs covering return grilles all restrict airflow. Your AC needs a minimum volume of air moving across the coil to function. Open every vent in the house, clear obstructions from returns, and make sure the return grille filter (if present) isn't clogged.

Open the air handler access panel (the panel near the filter slot on the indoor unit). If the evaporator coil — the A-shaped aluminum fin assembly — is coated in frost or ice, the system is freezing up. Turn the system OFF but leave the fan set to ON to thaw the ice (1–4 hours depending on severity). A frozen coil is caused by low airflow (dirty filter, blocked vents) or low refrigerant. If it freezes again after thawing with a clean filter, call an HVAC technician — you likely have a refrigerant leak.

Your AC system uses two breakers: one for the indoor air handler/furnace and one for the outdoor compressor. A tripped breaker sits in the middle position (not fully ON or OFF). Flip it all the way OFF, then back ON. If it trips again immediately, do NOT keep resetting it — there's an electrical fault that needs professional diagnosis.

There's a disconnect box mounted to the wall within 6 feet of the outdoor unit. Open it — some have a pull-out block, others have a breaker switch inside. Make sure the pull-out block is inserted correctly (not upside down or loose) and any internal breaker is ON. These disconnects get bumped by landscapers, kids, and contractors working on the exterior.

Most AC systems have a safety float switch on the condensate drain line. When the drain clogs and water backs up, the float switch shuts down the system to prevent water damage. Look for a small switch or sensor on the drain line near the indoor unit. If the drain pan is full of water, the drain is clogged. Clear it by pouring a cup of vinegar into the drain line access port, or use a wet/dry vacuum on the outdoor drain terminus to pull the clog out.

The outdoor unit needs at least 24 inches of clearance on all sides for airflow. Cut back shrubs, pull weeds, and remove leaves, grass clippings, and debris from the condenser fins. A unit choked by overgrown landscaping runs 10–20% less efficiently and can overheat on hot days. Spray the fins with a garden hose (gentle pressure, not a power washer) to clear embedded dirt and pollen.

With the thermostat calling for cooling, go outside and look at the top of the condenser. The large fan should be spinning. If you hear humming but the fan isn't moving, the fan motor or capacitor has failed — the unit will overheat and shut down on safety. Turn the system off immediately to prevent compressor damage. This requires a service call.

The compressor is the large cylinder inside the outdoor unit. When running, it produces a steady low hum. If you hear clicking, buzzing, or nothing at all, the compressor isn't running. Common causes: failed capacitor ($150–$300 repair), failed contactor ($150–$400 repair), or compressor failure ($1,500–$3,000 repair). Any compressor issue requires a licensed HVAC technician.

The large insulated copper line (suction line) carries cold refrigerant from inside to outside. In normal operation, it should feel cold to the touch and have condensation (sweating) on the insulation. If it's warm or room temperature, the system isn't moving refrigerant properly — likely low charge from a leak, or a failed compressor. Report this symptom to your HVAC technician.

A hissing sound near the indoor or outdoor coil or along the refrigerant line set indicates a refrigerant leak. Bubbling sounds suggest air has entered the system through a leak. Refrigerant leaks get worse over time, not better. The system gradually loses cooling capacity as charge drops. This requires professional leak detection and repair — $200–$600 for most leak repairs plus $50–$150 per pound of refrigerant to recharge.

Hold a thermometer at a supply vent (cold air) and at the return grille (warm air). The difference should be 15–20°F. If it's less than 12°F, the system is underperforming. Less than 8°F with a clean filter and adequate airflow strongly suggests low refrigerant, a failing compressor, or a severely dirty evaporator coil. This data helps your HVAC tech diagnose the problem faster.

Repeated breaker trips indicate a short circuit or ground fault — continuing to reset risks fire or compressor damage. A burning smell means an electrical component is overheating. Ice on the outdoor unit (not the indoor coil) indicates a severe refrigerant issue. All three warrant same-day service.

These symptoms indicate declining performance rather than immediate danger. The system needs professional diagnosis — refrigerant charge check, capacitor test, or coil cleaning. Provide the tech with your observations: supply/return temperature delta, filter condition, outdoor unit status, and any sounds you noticed.

The model number is on a label on the outdoor unit and indoor air handler. The installation date is often on the same label or on the furnace. This information helps the tech arrive with the right parts and quote accurately. Systems over 15 years old may not justify expensive repairs — knowing the age helps the repair-vs-replace conversation.