Why Your AC Isn't Cooling: A Diagnostic Checklist for Minnesota Homeowners

It's 90°F and humid in Minneapolis, and your AC isn't keeping up. Before calling for emergency service, work through this checklist — many AC problems have straightforward causes a homeowner can address without a technician.

Step 1: Verify the Basics

Is the thermostat set correctly? Confirm it's in COOL mode, not FAN or HEAT. Set the setpoint at least 5°F below current indoor temperature. Is the air handler/furnace running? You should hear the indoor blower running when the AC is calling. If the indoor unit isn't running, check the circuit breaker — there are typically two: one for the outdoor unit and one for the indoor air handler/furnace. Is the outdoor condensing unit running? Go outside and verify the fan on top of the outdoor unit is spinning when a cooling call is active. If the outdoor unit isn't running but the indoor blower is, the outdoor unit may have tripped its own breaker or disconnect.

Step 2: Check the Filter

A severely clogged air filter restricts airflow across the evaporator coil. Restricted airflow causes the coil to freeze — ice forms on the coil, further blocking airflow, and cooling stops. Replace the filter and check if there's ice visible on the refrigerant lines entering the air handler (frost or ice on the larger copper line indicates freezing). If frozen, turn the system to FAN ONLY (no cooling) for 2–4 hours to thaw, then restart with new filter. See our filter replacement guide.

Step 3: Check the Outdoor Unit

Is the condenser fan running? If the fan isn't spinning but you can hear the compressor humming, the fan capacitor may have failed. This is a common, inexpensive repair ($15–$60 for the part). See our capacitor guide. Is the outdoor unit surrounded by debris? Grass clippings, leaves, or other debris packed around the condenser coil restrict airflow and reduce efficiency significantly. Clear a 2-foot clearance around the unit. Are the condenser coil fins visibly dirty? Dirty condenser fins are a major efficiency killer. See our coil cleaning guide.

Step 4: Assess the Cooling Output

If everything is running but cooling seems inadequate: measure the temperature of air coming from supply registers. It should be 15–20°F cooler than the return air temperature. If supply air is only 5–10°F cooler than return, possible causes include low refrigerant charge (leak), dirty evaporator coil, or a failing compressor. Check if the larger copper refrigerant line (the suction line) entering the outdoor unit feels cold and has some condensation on it — it should. If it's warm, refrigerant charge may be low.

Step 5: Check for Ice on the Indoor Coil

A frozen evaporator coil is one of the most common causes of an AC that's running but not cooling. Causes: dirty filter, low refrigerant, very low outdoor temperature (below 60°F — AC doesn't work well in very cool weather), or restricted airflow from closed registers.

To check: find the refrigerant lines where they enter the air handler — the larger line (suction line) should be cool but not iced. Ice visible on the line or on the coil itself confirms freezing. Turn to FAN ONLY to thaw (at least 2–4 hours, sometimes up to 12 hours for severe icing). Don't run AC cooling during thaw.

Step 6: When to Call a Technician

Call a professional if: the system has no refrigerant (must be checked and charged by an EPA 608 certified tech), the compressor is failed (won't start, makes unusual grinding noise), there's a refrigerant leak (requires leak detection and repair before recharge), the electrical components beyond capacitors need testing.

When AC Problems Mean Replacement

If your AC is 15+ years old, uses R-22 refrigerant, or has a failed compressor, replacement is usually more economical than repair. A Goodman central AC system from Furnace Direct at wholesale pricing gives you modern R-454B refrigerant, 10-year warranty, and full efficiency for years of reliable cooling.

Browse replacement AC equipment at furnace.direct/collections/cooling.

Related reading: AC Capacitor Guide | AC Coil Cleaning Guide | Signs Your AC Needs Replacement