AC Not Blowing Cold Air: The Checks in Order

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

AC Not Blowing Cold Air: The Checks in Order

Short answer

Put a thermometer in the return grille and another in a supply vent and take the difference. A working system drops the air by 16 to 22 degrees. Under twelve means low refrigerant or a filthy coil, over twenty two means not enough air is moving, and no drop at all usually means the outdoor unit is not running.

An AC not blowing cold air is worth measuring before it is worth guessing at. Hands are poor thermometers, and the difference between a system that is struggling and one that has stopped is a number you can take in five minutes.

A healthy air conditioner makes the air about 16 to 22 degrees colder than the air going back into it. Everything below hangs off that one reading.

Before you start

What to carry: two cheap thermometers, a torch and a new filter. Ten minutes for the reading.

Gas or electric? The cooling half is the same either way, but the indoor box differs. A gas furnace has a gas pipe and a flue, with the cooling coil sitting on top of it. An electric air handler has a thick cable and no flue. Both have the same filter, the same drain pan and the same outdoor unit.

Check the thermostat first. Set to Cool rather than Fan or Off, the fan on Auto rather than On, and the temperature pushed a good five degrees below the room. Then wait five minutes, because every system holds the compressor off for a few minutes after any change.

Three things out there that are not do it yourself. The capacitor in the outdoor unit can hold a charge after the power is off, and it will give you a serious belt. Clearing leaves from the outside is fine. Opening it up is not.

The refrigerant. An air conditioner does not use it up, so if it is low there is a leak. Topping it up without finding the leak is money out of the window, and in the United States venting it is against the law.

A frozen coil. Never keep running the cooling on a system that has iced up. Liquid refrigerant reaching the compressor is what turns a cheap fault into a new system.

Measure the drop

The honest test is the difference between the two, not what the thermostat says the room is.

Check 1Free, ten minutes

One thermometer in the return, one in a supply vent

Let it run for ten minutes first so you are not reading the air that was already sitting in the ducts.

Put one thermometer in the big return grille and the other in a supply vent, ideally one near the indoor unit. Take the difference.

Sixteen to twenty two degrees colder at the supply is a system doing its job, and on a very hot day that still may not get the house down to where you want it. That is a size and a weather problem rather than a fault.

Under twelve degrees points at low refrigerant or a coil thick with dirt. Over twenty two means the air is moving too slowly across the coil, which sends you to the filter. And no drop at all means nothing is cooling, which is the next section.

No drop at all: is the outdoor unit running?

Check 2Free to check

Go outside while it is calling for cooling

With the thermostat set low and the indoor fan blowing, the outdoor unit should be humming with its fan turning.

Silence means power. There are two breakers in the main panel, one for the indoor unit and one for the outdoor unit, plus a pull out disconnect block on the wall beside the unit that gets left out or put back upside down.

A hum with the fan sitting still is usually the capacitor. A fan turning with no deeper drone underneath it is the contactor or the compressor resting on its own overload.

It is the same box and the same checks as a heat pump, so the detail is on that page: the breaker, the disconnect and the capacitor, in order.

The drain pan nobody checks

A blocked drain fills the pan, the float lifts, and the cooling stops, sometimes with the fan still running. Nothing is broken.

Check 3

Look at the drain pipe and the pan under the indoor coil

Cooling makes water, a lot of it, and it runs off the indoor coil into a pan and out through a small plastic pipe.

That pipe grows slime and blocks. The pan fills, a small float switch in it lifts, and that switch cuts the cooling to stop the pan overflowing into the ceiling.

RectorSeal, which makes these switches, says one wired into the red thermostat wire shuts down the entire unit, and one in the yellow wire leaves the fan running with no cold in the air. Either way it reads exactly like a dead air conditioner.

Look for standing water in the pan, or water stains under the indoor unit. Clearing the pipe, usually with a wet vacuum at the outside end, drops the float and everything starts working again.

Ice on the coil or on the copper pipe

Check 4Free, a few hours

Switch the cooling off and run the fan only

If you can see ice on the indoor coil, or frost on the big insulated copper pipe at the outdoor unit, the system has frozen up and it will not cool anything until it has thawed.

Set the thermostat to Off for cooling and the fan to On. That pushes room air across the coil and melts it, and Bryant says it can take anywhere from a few hours to a whole day.

Then find out why. Ice means either not enough air across the coil, which is the filter, the vents or the blower, or not enough refrigerant in it. Those two are told apart by the drop you measured at the top of this page: too much drop is airflow, too little is refrigerant.

Do not run the cooling again until the ice is gone and the cause is dealt with.

The things that choke it

The filter. Hold it up to a light. No light through it and that is your fault, and it is the cause of most frozen coils.

Closed vents and blocked returns. Every vent open, including the rooms nobody uses, and the furniture off the return grille.

The outdoor coil. It sheds the heat it took out of the house, and it cannot do it through a mat of grass clippings and cottonwood. Power off at the disconnect, then rinse it from the inside outwards with an ordinary garden hose. Never a pressure washer, because it folds the fins flat.

The indoor coil. Behind the filter, and if the filter has been left too long the dirt is on the coil instead. That one is a service call.

Ductwork in a hot attic. Cold air that travels through a hundred degree attic in a leaky duct arrives lukewarm even when the unit is perfect.

Questions people ask

It cooled fine last summer and barely works now.

A system that has slowly got worse over a season is usually losing refrigerant or has a coil that has silted up. Measure the drop, because the two give opposite readings.

How cold should the air actually feel?

Cold but not freezing. On a 95 degree day with 78 inside, a supply vent around 58 to 62 is right. People expect it to feel like a fridge and it never does.

The unit runs all day and never switches off.

On the hottest days that is normal and it is how a correctly sized system behaves. Running constantly and still losing ground is not, and that is where the drop measurement earns its keep.

Can I add refrigerant myself with a kit?

Kits exist and they cause more harm than good. Overcharging a system damages the compressor, and the gas is leaving through a hole that is still there.

Water is dripping from the indoor unit.

Blocked drain, same as the section above, and worth dealing with quickly because the next thing it does is come through a ceiling.

Mine is a ductless wall unit rather than ducted.

Then the filters are the first thing and they are inside the head itself: why a mini split stops cooling.

Sources: DOE Building America "Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement" (temperature split, low split for low capacity, high split for low airflow), Bryant "What Temperature to Set Air Conditioner in Summer" (about 20°F below the return air, 78 in and about 58 at the vent), Carrier service manual 24-25-9SM (capacitors, 5 minute cycle protector), Carrier "Troubleshoot an Air Conditioner" (panel breaker and the disconnect by the unit), Trane "Troubleshooting the HVAC Air Handler Overflow Switch" and the RectorSeal Safe-T-Switch SS2 instructions (float switch wiring), Lennox "How to Clean Air Conditioner Fins" and Trane "How to Clean Condenser Coils" (garden hose, no pressure washer), Bryant "Why Does My AC Keep Freezing?" (a few hours to a day to thaw), and the federal ban on venting refrigerant in EPA "Phasing Out HCFC Refrigerants to Protect the Ozone Layer" and Carrier 25HBC/25HCC/25HCE installation instructions