Furnace Blowing Cold Air: What It Is Telling You

By Ray Bennett · Updated 2 October 2026 · 4 min read · Checked against the sources at the end

Furnace Blowing Cold Air: What It Is Telling You

Short answer

Cold air all the time between heat cycles is usually the fan switch left on ON instead of AUTO, which is free to fix. Warm air that turns cold part way through is an overheating furnace, and that starts with the filter. Cold air with the burners lighting and dying after five seconds is the flame sensor.

A furnace blowing cold air is not one fault, it is four, and the one you have depends on when the air turns cold. Somebody who fixes the wrong one buys a part they did not need.

So the first question is not what is broken. It is: cold all the time, cold at the start of every cycle, or warm for a few minutes and then cold?

Before you start

What to check first: the thermostat, the filter and the switch on the side of the furnace. All three are free, and between them they cover a lot of these.

What to carry: a torch and a new filter if yours has not been changed in months. Ten minutes.

Is it a furnace or a heat pump? If the box outside runs in winter and the indoor unit has no gas pipe, you have a heat pump, and lukewarm air in cold weather is normal behaviour for one rather than a fault.

Power and gas off before the panel comes off The service switch on the side of the furnace, or the breaker. There is 120 volts and a gas valve behind that door. Watching one cycle, further down, is done with the panel on and the sight window.

Cold air all the time, even between heat cycles

This is the free one, and it is worth ruling out first.

Look at the thermostat for a switch or a setting marked FAN, with the choices ON and AUTO. On ON, the blower runs day and night, pushing room temperature air through the vents whether the furnace is burning or not. Air at 68 degrees feels cold on your hand.

Set it to AUTO. The fan now runs only when there is heat to move.

If it is already on AUTO and the air still never warms, keep going.

Watch one cycle, it tells you the rest

Turn the thermostat up and watch through the sight window. Where the cycle stops is the answer.

Turn the thermostat up five degrees and stand at the furnace with the light off so the window shows up.

The four faults, by what you saw

Check 1

Burners light, then go out after about five seconds

The furnace fires properly, runs a few seconds and shuts the gas off again, and the blower carries on pushing air that is now cold.

That is the control board saying it cannot feel the flame, and often it is a coated sensor rod: how to clean a furnace flame sensor.

Check 2

Warm air for a few minutes, then cold, over and over

The furnace is overheating. Its limit switch shuts the gas off to protect the heat exchanger and leaves the blower running to cool it down, which is the cold air you feel.

Pull the filter out and hold it up to a light. If you cannot see light through it, that is your fault, and a new filter is cheap. Then open every vent in the house and pull the furniture off the return grille, because a furnace starved of air overheats exactly the same way.

Left alone, this one cracks heat exchangers, which is the expensive end of the trade.

Check 3

Nothing lights at all, and there is no pilot flame

If your furnace has a knob marked OFF, PILOT and ON, and the little flame is out, nothing else will happen until it is relit: how to light a furnace pilot safely.

Check 4

Nothing happens at all, not even a fan

Thermostat batteries first, then the switch on the side of the furnace, which looks like a light switch and gets turned off by decorators and by people who mistake it for one. Then the breaker.

Some furnaces also refuse to start with the front door off or not clicked shut, because there is a switch behind it.

The high efficiency ones have one more

If your furnace has white plastic pipes going outside rather than a metal flue, it makes water while it runs, and that water has to drain away.

When the condensate trap or the pump blocks, a pressure switch stops the burners lighting. Some models flash a code for it. Pouring a cup of warm water through a blocked trap can be the whole repair, and a pump that has died can be replaced.

Questions people ask

Cold air for the first minute, then it warms up. Is that a fault?

No. Plenty of furnaces start the blower before the metal is hot. Anything under a minute or two is normal.

It only does it on the coldest nights.

A furnace running non stop shows up a marginal flame sensor or a half blocked filter that copes the rest of the year.

Some rooms are warm and some are cold.

That is not this fault. That is airflow balance, or a duct that has come apart in the crawl space.

The furnace is flashing a code at me.

Read it against the label inside the door, not against a code list from another make. They all use different patterns.

Can I keep using it while it does this?

The overheating one, no. A furnace tripping its limit switch again and again is the one that cracks a heat exchanger, and that is the repair nobody wants.

Sources: Carrier 58SC0A/58SC1A instructions (sequence of operation, limit and flame codes, blower door switch), Goodman *MSS9 (GMSS96) installation instructions, White-Rodgers 50M56U-843 instructions (timings), York installation manual 561276-UIM-A-0310 (limit and drain codes), and the U.S. Department of Energy Energy Saver Guide (2017), fan switch on Auto