If your AC is running but not blowing cold air, check four things first: the thermostat setting, the air filter, the breaker and outdoor disconnect, and the condensate drain. If all four are fine, or you find ice on the indoor coil, a silent outdoor unit or a burning smell, switch the system off and call a technician.

What you notice	Likely cause	Fix it yourself?
Room-temperature air, outdoor unit quiet	Thermostat on fan or heat, tripped breaker, disconnect off	Yes: check settings, reset the breaker once
Weak airflow, house slowly warming	Clogged filter or blocked returns	Yes: replace the filter
System shut off, water in the drain pan	Blocked condensate drain tripped the float switch	Sometimes: clear the outside end of the line
Ice on the copper lines or indoor coil	Frozen coil from low airflow or low refrigerant	Partly: thaw it, then call
Outdoor unit hums or clicks, fan not spinning	Failed capacitor or contactor	No: electrical hazard
Runs nonstop, air only mildly cool	Dirty condenser, refrigerant leak, failing compressor, leaky ducts	Rinse the outdoor coil; the rest needs a technician

Not every air conditioning problem needs a technician. A fair number of the no-cooling calls we take in a Houston summer turn out to be one of four things a homeowner can check in ten minutes, and knowing which is which can save you a visit and, in August, a wait.

What should I check first when my AC is not blowing cold air?

1. Is the thermostat actually asking for cooling?

Check that it is set to cool, not heat or off, and that the set temperature is below what the room reads. It sounds obvious, and it is regularly the answer, particularly after a power cut, which can return some thermostats to a default schedule nobody chose.

Look at the fan setting too. On on instead of auto, the blower runs between cooling cycles, so the vents feel warm and muggy even when the system is cooling normally. On a battery model with a dim, blank or erratic display, replace the batteries first. If the settings are right and nothing happens, Carrier suggests a reset: thermostat off, AC breaker off, wait at least five minutes, breaker on, then thermostat back to cool.

2. When did you last change the air filter?

A clogged filter is the single most common cause of poor cooling, and here filters load faster than the box says, because a Houston system runs most of the year. Restricted airflow makes the system work harder, cool less and eventually freeze the evaporator coil, at which point you get warm air from the vents and a puddle near the air handler.

Hold the filter up to a light. If you cannot see light through it, change it. ENERGY STAR recommends checking it every month. After weeks on a choked filter, give it a few hours before judging; a frozen coil needs time to thaw.

3. Has the breaker or the outdoor disconnect tripped?

A central AC usually has two power points: a breaker in the panel and a disconnect box on the wall by the outdoor unit. Check both. A breaker that has tripped once can be reset, and if it holds, it may simply have been a surge.

If it trips again, stop. A repeatedly tripping breaker means something is drawing more current than it should, and resetting it over and over risks the compressor and the wiring. That is a call, not a fix. Do not go beyond flipping the switch or pulling the disconnect handle; there is line voltage inside.

4. Is the condensate drain blocked?

This is the one most people have never heard of and the one that causes the most damage. Your system pulls a great deal of water out of Houston air and sends it down a drain line. When that line blocks with algae, a float switch shuts the system off. That is deliberate: the system is protecting your ceiling, not failing.

Look for standing water in the drain pan or a drip at the outside end of the line that has stopped. Clearing it is sometimes a wet-dry vacuum job at the outside end, and sometimes not. If it blocks again within weeks, or water has reached drywall, have it looked at.

Why is my AC running but not cooling if those four are fine?

Then the cause is usually in the refrigerant circuit, the coils or the parts that start the outdoor unit.

Is the evaporator coil frozen?

Frost on the copper lines or indoor coil, or water around the air handler, means the coil has frozen. Ice forms when too little warm air crosses the coil (dirty filter, dirty coil, blocked returns, weak blower) or the refrigerant charge is low. Once it starts, the ice insulates the coil and it snowballs. To thaw it safely:

1. Switch the thermostat from cool to off.
2. Set the fan to on, not auto. Trane notes this speeds thawing; expect a few hours.
3. Put towels near the air handler and watch the drain pan.
4. Replace the filter while it thaws.

Do not chip at the ice; Trane warns that can damage the coil and cause a refrigerant leak. If it refreezes after a clean filter and a full thaw, the problem is airflow or refrigerant, and it needs measuring.

Could it be a refrigerant leak?

Refrigerant is not used up like fuel. If the charge is low, it is leaking. Signs include ice on the lines, a hissing sound, oily residue on line fittings, and a system that runs constantly without reaching temperature.

This is not a homeowner job. Under EPA rules issued under Section 608 of the Clean Air Act, technicians who service equipment that could release refrigerant must be certified, and refrigerant for home systems can only be sold to certified technicians. The real fix is finding and repairing the leak, then charging to the manufacturer's specification. Topping up without a repair just means the same call next summer.

Is the outdoor unit dirty or blocked?

The outdoor condenser sheds the heat pulled out of your house. Packed with grass clippings and dust, or boxed in by shrubs, it cannot, so the house never quite gets cold. With the power off at the disconnect, clear debris, trim plants back and, as Carrier suggests, gently rinse the fins with a garden hose. No pressure washer: bent fins block airflow too.

Is it a capacitor, contactor or compressor problem?

If the outdoor unit hums and clicks but nothing starts, or its fan is not spinning, a failed capacitor or a contactor that is not pulling in are the usual suspects. Left alone, a weak capacitor makes the compressor strain on every start, one of the ways compressors die early. Leave these parts alone: capacitors can hold a dangerous charge even after the power is off.

If the outdoor unit runs but the air is only ever mildly cool, or it trips the breaker under load, the compressor may be failing. Our guide to when a compressor failure means replacing the whole system walks through that decision.

Are leaky ducts losing the cold air?

Sometimes the system makes cold air and the house just is not getting it. ENERGY STAR estimates a typical house loses about 20 to 30 percent of the air moving through its ducts to leaks, holes and poor connections. In Houston most ductwork runs through a hot attic, so a crushed or disconnected run dumps cold air where it does no good.

Is the system too small or too old for the house?

If the AC has never kept up on the hottest afternoons, it may not be a fault at all. An undersized or aging system runs for long stretches and still falls behind, and rooms furthest from the air handler suffer first. If the system is more than a decade old and this is not the first failure of the season, have the repair-or-replace conversation now rather than in the next heat wave. Our guide to buying a new AC unit in Houston covers that choice.

Why does Houston weather make this worse?

The cooling season is long, so filters, drains and coils load up faster than generic schedules assume. And much of an AC's work here is removing moisture. A system short on refrigerant or airflow often loses its ability to dry the air before it stops cooling outright, so the house feels clammy at the set temperature. If it cools but still feels damp, whole-home dehumidification may be the answer rather than a repair.

When should I turn the AC off and call right away?

• You smell burning or hot plastic.
• The breaker trips again after one reset.
• There is ice on the refrigerant lines or indoor coil.
• Water is dripping through a ceiling or pooling near the air handler.
• The outdoor unit is grinding or screeching.

In each case, running the system longer usually makes the repair bigger.

What will a technician actually check?

A diagnosis is measurements, not guesses: the temperature split across the indoor coil, refrigerant pressures against the manufacturer's chart for the day's conditions, amp draw on the compressor and fan motors, capacitor and contactor condition, coil condition and static pressure, and the condensate drain and float switch. Then you should hear what needs doing and what does not. Regular maintenance visits cover most of this before summer.

Frequently asked questions

Why is my AC running but not blowing cold air?

Most often a thermostat setting, a clogged filter, a tripped breaker, a blocked condensate drain, a frozen coil or a refrigerant leak. Check the first four yourself; the rest need a technician.

Should I turn my AC off if it is not cooling?

Yes, if you see ice, smell burning or the breaker keeps tripping. Running it in those conditions can turn a small repair into a compressor replacement.

How do I unfreeze my AC?

Turn cooling off, set the fan to on, replace the filter and let the coil thaw completely, usually a few hours. Never chip at the ice.

Why does my AC keep freezing up?

Repeat freezing means low airflow or low refrigerant. If a clean filter and open returns have not stopped it, a dirty coil, weak blower or leak is likely, and a technician needs to measure it.

Can I add refrigerant to my AC myself?

No. EPA rules under Section 608 of the Clean Air Act limit refrigerant sales to certified technicians. Low refrigerant also means a leak, so adding more does not fix the problem.

Can a dirty filter really stop an AC from cooling?

Yes. Restricted airflow is one of the most common causes of a frozen coil and poor cooling, and it is the easiest fix on this list.

Still not cooling? Talk to Ross Air

If you have worked through the four checks and the house is still warm, Ross Air diagnoses and repairs all makes and models, with upfront pricing before any work starts and 24/7 emergency service. We work across Houston and the southwest suburbs, including Katy, Sugar Land, Missouri City, Stafford, Richmond and Rosenberg. Read more about our HVAC repair service or get in touch.

Call Ross Air: (281) 223-1176