AC Not Cooling? Diagnostic Guide

Before pulling panels, verify the thermostat is actually calling for cooling — wrong mode, a mis-set setpoint, or a tripped transformer will make the whole system look like it's not working.

• Confirm the thermostat is set to Cool mode, not Fan or Off.
• Check the setpoint — if it's 80°F and the house is 79°F, the system won't call for cooling at all.
• Check for a blank or flickering display — a blank screen usually means a power problem (breaker, fuse, C-wire, or failed 24V transformer). See our thermostat blank screen guide for the full 24V transformer diagnostic sequence.
• At the air handler, verify 24V between the Y and C terminals at the control board while the thermostat calls for cooling. No 24V = no cooling signal — trace back through the thermostat wiring or check the transformer.

Go outside. The condenser unit needs unrestricted airflow across the coil to reject heat. If it's blocked — by debris, vegetation, or a wooden fence too close — the system loses cooling capacity and may blow warm.

• Check that the disconnect box near the outdoor unit is fully engaged — vibration loosens the fuse block over time, causing intermittent or no power to the compressor.
• Confirm the condenser fan is spinning. If it's not running at all, check the breaker. If the breaker trips repeatedly, stop and call a tech — there's a hard fault.
• Inspect the coil face — if it's caked with cottonwood, dirt, leaves, or grass clippings, rinse it with a garden hose from the inside out (never use a pressure washer — it bends fins and worsens the problem).
• Verify at least 24 inches of clearance around the unit on all sides — vegetation too close reduces airflow dramatically.
• If the coil is clean, the fan spins, the disconnect is engaged, and you still have warm air — see AC Blowing Warm Air for the parallel symptom guide covering compressor, refrigerant, and capacitor diagnostics.

A frozen coil is a symptom, not a root cause. Open the air handler access panel and look at the evaporator coil. If you see ice or heavy frost — on the coil itself or on the large copper suction line going into the air handler — the coil is frozen.

• Immediately set the thermostat to Fan Only — do not run cooling while the coil is frozen.
• Leave the fan running for 2–4 hours to defrost. Check progress by looking at the coil through the access panel.
• Once fully defrosted, examine the coil. Is it dirty? Is the filter clogged? Is there a blocked return vent? These are the three root causes of a frozen coil. Also check for restricted airflow — closed registers, blocked returns, or a dirty filter all produce the same freeze pattern.
• After addressing the root cause (filter replaced, return cleared, coil cleaned), restart the system in cooling mode and monitor. If the coil re-freezes within 24 hours, move to Step 6 (refrigerant) or Step 5 (capacitor).

If the condenser fan runs but the compressor hums and doesn't start — or starts and quickly shuts off on thermal overload — the capacitor is the first thing to check. Capacitors are the most common component failure on residential AC units over 5 years old.

• Shut off power at the disconnect box. Wait 5 minutes for the capacitor to discharge. Do not skip this — capacitors hold lethal charge even with power off.
• Locate the capacitor — it's usually in the outdoor unit's electrical compartment, a cylindrical can with spade terminals. It will have a label showing MFD (microfarads) and voltage rating (370V or 440V).
• Use a multimeter with a capacitance setting. Touch the meter leads to the two main terminals (not the common terminal). Compare the reading to the rated MFD on the label.
• Acceptable range is ±6% of rated MFD. Below that, the capacitor is failed and needs replacement. Never replace a capacitor with a lower voltage rating — always match or exceed it.
• Capacitors are marked with dual ratings (e.g., 45/5 MFD) — the higher value is for the compressor run winding, the lower for the condenser fan.

For capacitor and control board fault patterns shared with furnace ignition failures, see also our Furnace Not Igniting guide — the hard-lockout patterns and startup diagnostics are directly applicable.

If the filter is clean, the coil isn't frozen (or re-freezes after defrost with a clean filter), the capacitor is good, and the condenser coil is clean — you're looking at a refrigerant issue. You cannot check refrigerant charge without manifold gauges, and you cannot legally handle refrigerant without EPA 608 certification.

• Connect manifold gauges to the service ports (suction side = large copper line, liquid side = small copper line). With the system running, compare readings to manufacturer specifications.
• Signs of low refrigerant: suction pressure well below spec, ice on the suction line, warm air from vents despite the system running, hissing or bubbling sounds near the refrigerant lines.
• Measure the temperature difference between supply and return air (delta-T). Healthy is 14–22°F. If it's only 4–8°F, the system is not removing heat effectively — low charge is a likely cause.
• If you find a leak: you must find and fix it before recharging. Adding refrigerant without fixing the leak is a temporary fix — it will leak out again, and refrigerant is expensive.

If everything above checks out — thermostat calling, capacitor good, refrigerant at spec — but the compressor still won't run, the problem is in the control circuit: the contactor or the control board.

• Contactor: The contactor is a electrically-controlled switch that sends 230V to the compressor when the 24V coil is energized. Over time, the main contacts pit from arcing — you can see pitting and burn marks on the contact surface. A pitted contactor won't close fully, so the compressor never receives power. Replacement cost: $30–80 parts, $80–150 with labor.
• 24V coil failure: The coil on the contactor (a small coil next to the main contacts) can fail — open winding. Test with a multimeter: expecting 24V coil resistance around 20–60 Ω. Open circuit = failed coil.
• Control board hard-lockouts: Some systems display a fault code on the board when they detect a fault (open compressor circuit, missing delta-T, high-pressure switch trip). If the board is locked out, the status light on the board will show a repeating error code — look it up in the service manual or search for the model number + fault code.
• If the contactor is good, the 24V coil is good, and the board isn't in hard-lockout — the compressor winding is likely open. Time to discuss replacement options with the customer.