A furnace that runs but blows cold air is one of the most common winter service calls, and a meaningful share of them get resolved by the homeowner before the tech arrives. The failures range from a thermostat setting anyone can fix in ten seconds to a cracked heat exchanger that means replacing the furnace. This guide walks the eight causes in the order a technician actually checks them, tells you which ones are safe to handle yourself, and gives real 2026 repair pricing so you know whether the quote you are looking at is fair.
Before anything else, confirm the thermostat fan setting is on AUTO, not ON. With the fan set to ON, the blower runs continuously and pushes room-temperature air between heating cycles — which feels exactly like a broken furnace and is not one. This single setting explains a surprising number of service calls. While you are there, verify the thermostat is in HEAT mode and the setpoint is at least three degrees above the current room temperature.
Next, check the air filter. A filter clogged solid restricts airflow enough that the furnace overheats, trips its high-limit switch, and shuts the burners down while the blower keeps running to cool the heat exchanger — again, cold air from the vents. Filters are $8 to $25 and should be changed every 60 to 90 days. Finally, confirm the furnace has power at the breaker and that the service switch on the wall beside it (it looks like a light switch) is on.
On a modern furnace, the most common genuine failure is a dirty flame sensor. This is a thin metal rod in the burner flame that proves combustion is happening. Over a few seasons it builds an oxide film, stops conducting, and the control board shuts the gas off two to seven seconds after ignition as a safety measure. The symptom is distinctive: the furnace tries, you hear ignition, then it stops and retries a few times before locking out. Cleaning it costs $90 to $180 on a service call; replacement is $150 to $300.
Igniters fail too. A hot surface igniter is a fragile silicon carbide element that glows orange to light the gas, and it has a service life of about five to seven years. If the furnace never ignites at all and you see no glow through the inspection port, that is usually the igniter — $180 to $400 installed. On older furnaces with a standing pilot, a pilot that will not stay lit is typically a bad thermocouple at $100 to $200.
If the furnace is clearly firing but the air arriving at the registers is lukewarm, the problem is usually airflow or distribution rather than heat production. A failing blower motor or capacitor moves too little air; a bad run capacitor is a $150 to $350 repair while a full blower motor replacement is $400 to $900. A blower wheel packed with dust — common in homes that have gone years without a filter change — can cut airflow by a third and is cleaned for $150 to $300.
Duct problems are the quiet culprit. Disconnected or leaking ducts in an unconditioned attic or crawlspace dump heated air where nobody lives, and the vents furthest from the furnace go lukewarm first. Sealing accessible duct joints with mastic runs $400 to $1,500 depending on how much of the system is reachable, and it usually pays back in reduced runtime within a few seasons.
High-efficiency condensing furnaces produce acidic water as a byproduct, and that water drains through a trap and a small pump. When the drain clogs, a float switch cuts the furnace off — often with no error code that means anything to a homeowner. Clearing a condensate line is $100 to $250; a new condensate pump is $150 to $350. If you see water pooling near the base of the furnace, this is almost certainly your problem.
Blocked exhaust is the more serious version. The pressure switch verifies that the inducer fan is actually venting combustion gases outside. A bird nest, a snow drift over a sidewall vent, or a cracked inducer hose will keep the furnace from firing at all. Clear obvious blockages at the exterior vent yourself; anything inside the cabinet needs a tech. Never bypass or jumper a pressure switch — it exists to keep carbon monoxide out of your house.
A cracked heat exchanger is the failure that ends a furnace's life. It allows combustion gases, including carbon monoxide, to mix with the air you breathe, and a tech who finds one is required to red-tag the unit and shut off the gas. Symptoms include a persistent soot smell, visible flame rollout, a yellow rather than blue flame, and CO detector activation.
Replacing a heat exchanger costs $1,500 to $3,500 in parts and labor, which on any furnace over twelve years old is poor value against $4,500 to $8,500 for a new high-efficiency unit with a fresh warranty. If your furnace is under warranty — many carry 20-year or lifetime heat exchanger coverage — the part may be free and you only pay labor, which changes the calculation entirely. Check your warranty before agreeing to anything.
The single most common cause is the thermostat fan setting left on ON instead of AUTO, which runs the blower continuously between heating cycles. If the fan is already on AUTO, the next likeliest causes are a clogged filter tripping the high-limit switch or a dirty flame sensor shutting the gas off seconds after ignition. Both are inexpensive fixes.
Yes, if you are comfortable working around a gas appliance. Turn off power at the service switch and gas at the valve, remove the single screw holding the sensor rod, scuff the metal lightly with fine steel wool or an emery cloth — never sandpaper grit that leaves residue — and reinstall it. Ten minutes and no parts cost. If the problem returns within weeks, the sensor or board needs professional attention.
Most common repairs land between $150 and $600. A flame sensor cleaning is $90 to $180, an igniter is $180 to $400, a blower capacitor is $150 to $350, a condensate line clearing is $100 to $250, and a control board is $400 to $800. Diagnostic fees run $90 to $170 and are usually credited toward the repair.
Between 60 seconds and about three minutes on most systems. Modern furnaces intentionally delay the blower until the heat exchanger is up to temperature, so cold air at the very start of a cycle is normal. If the air is still cool after five minutes of continuous running, something is genuinely wrong.
Usually yes for a short period, since most cold-air causes are lockouts that exist to protect you. Stop immediately and ventilate if you smell gas, smell soot or burning, see a yellow flame instead of blue, or a carbon monoxide detector alarms. Those signal a combustion problem that needs an emergency service call and possibly the gas utility.
Replace when the unit is over 15 years old and facing a repair above roughly $1,500, when the heat exchanger is cracked, or when you have had three or more service calls in two winters. A new high-efficiency furnace runs $4,500 to $8,500 installed and typically cuts fuel use 15 to 30 percent over an old 80 percent unit.