Your AC runs but will not cool: a step-by-step diagnosis

An air conditioner that runs, sounds normal and still leaves the house warm is one of the most common summer calls, and it has a small set of likely causes. Working through them in order avoids the expensive route before the cheap one has been ruled out.

The sequence below starts with airflow, moves to the outdoor unit and only then reaches the sealed refrigerant circuit. That order matches how often each cause turns out to be the real one.

How an air conditioner moves heat outdoors

An air conditioner does not create cold; it collects heat from inside the house and dumps it outside. The indoor coil absorbs heat from the air passing over it, the compressor raises the refrigerant pressure, and the outdoor coil releases that heat to the outside air.

Every part of that loop depends on two things: a refrigerant circuit that is sealed and correctly charged, and enough airflow on both sides of the system. When either one is compromised, the symptom is nearly always the same — the house will not reach the setting.

The sealed circuit and why it never needs a top-up

A refrigerant circuit is a closed loop. If the charge is correct when the system is installed, it stays correct, so a system that is low on refrigerant has lost it somewhere. That is why a recharge on its own is a temporary measure rather than a repair.

A proper approach finds and fixes the leak, then evacuates and recharges to the manufacturer's specification by weight. Top-ups by pressure reading alone can hide a leak for a season and overcharge the system, which shortens compressor life.

Airflow first: a dirty filter mimics a refrigerant fault

When a house will not cool, the cheapest thing to check is airflow, and it is also the most common culprit. A clogged filter, a closed register or a collapsed flexible duct restricts the air the indoor coil needs and can freeze the coil solid.

A frozen coil then blocks its own airflow and the whole system appears to fail at once. Before anyone opens a gauge manifold, confirm the filter is clean, the returns are open and the blower is actually moving air.

• Clean or replace the filter at the start of summer.
• Clear grass, leaves and furniture away from the outdoor unit.
• Flush the condensate drain line before the humid months.
• Never keep running a system whose indoor coil has frozen over.

The condenser coil outside and its job

The outdoor unit has to reject heat into the air, which is harder when its coil is clogged with grass clippings, cottonwood or pet hair. A dirty condenser raises the system pressure and forces the compressor to work harder for less cooling.

Keep a couple of feet clear on all sides, trim vegetation back and rinse the coil gently from the inside out when it is visibly soiled. This is one of the few meaningful jobs a homeowner can do without tools, and it protects the most expensive component in the system.

Frozen coils and what ice really means

Ice on the indoor coil or the refrigerant line is always a symptom, never a cause. It means the coil surface is dropping below freezing, which usually comes down to restricted airflow or a low refrigerant charge.

The right response is to switch the cooling off and let the ice melt completely before running it again. Restarting a frozen system forces the compressor to work against a blocked coil and can turn a simple airflow fix into a compressor replacement.

Sizing, short cycling and the humidity problem

An oversized system blasts cold air, satisfies the thermostat quickly and shuts off before it has removed much moisture. The house feels clammy even though the temperature reads correctly, and the repeated start-stop wears the compressor.

Correct sizing comes from a load calculation that considers insulation, windows, orientation and duct condition — not from matching the old unit's label. A system that runs in longer, gentler cycles cools better and dehumidifies properly.

The condensate drain nobody thinks about

Cooling produces water, and that water has to leave through a drain line and trap. Algae, dust and debris build up in the line over a season, and a blocked drain eventually trips the overflow switch or drips into the ceiling.

Flushing the drain line and checking the trap at the start of summer takes minutes and prevents one of the most common water-damage calls. If the drain pan holds standing water, the line or the slope is already a problem.

What a spring tune-up should include

A good tune-up is a sequence, not a list of sales items. It starts with the filter and airflow, moves through electrical connections and capacitors, checks the refrigerant charge and coil cleanliness, and finishes by confirming the system holds temperature and drains properly.

Ask what was measured — superheat, subcooling, amperage, temperature split — rather than what was replaced. A tune-up that produces no readings is a cleaning, and a cleaning alone rarely finds a slowly developing fault.

Why the first hot week exposes every weakness

A cooling system that limped through a mild spring meets its real workload in the first hot week, when every marginal component is stressed at once. That is why problems cluster in early summer rather than appearing gradually.

A spring visit finds those marginal parts while the calendar is quiet. Waiting until the heat arrives means waiting for an appointment at the busiest moment of the year.

Temperature split: a simple health check

The difference between the air entering the return and the air leaving the supply is a quick indicator of cooling performance. A normal split sits in a well-known range, and a reading far outside it points to airflow or charge problems.

It is not a substitute for measuring superheat and subcooling, but it is a useful first look. A technician who records the split on every visit builds a trend that shows a drifting system long before it fails.