Your furnace is running but blowing cold air — here’s how to find the problem.
When your furnace turns on but only pushes unheated air through the vents, something in the ignition or heat exchange process has failed. The blower motor is working (you feel air), but the burners aren’t firing or aren’t staying lit. Here are the most common causes.
Common Causes
🔥 Dirty Flame Sensor MOST COMMON
The flame sensor is a small metal rod that sits in the burner flame. It tells the control board that gas is actually burning. When it gets coated with carbon buildup, it can’t detect the flame — so the board shuts off the gas valve as a safety measure. The burners light briefly (you might hear the ignition click and see flame for 3–5 seconds), then shut off. The blower keeps running, pushing cold air.
You can often fix this yourself by removing the sensor and cleaning it with fine sandpaper or steel wool. Or bring it in and we’ll test it.
🔥 Bad Ignitor
If the ignitor has cracked or burned out, the gas won’t light at all. You’ll hear the inducer motor start, but no ignition click or glow. The control board waits for ignition, doesn’t get it, and locks out — the blower runs to clear any residual gas, pushing cold air. Hot surface ignitors are fragile and have a limited lifespan (typically 3–7 years).
🚨 Tripped High-Limit Switch
The high-limit switch is a safety device that shuts off the burners if the heat exchanger gets too hot. A dirty filter, blocked return, or failing blower motor can cause overheating. When the limit trips, the burners shut off but the blower keeps running to cool things down. Check your filter first — a clogged filter is the most common reason the limit trips.
⚙️ Gas Valve Not Opening
The gas valve controls gas flow to the burners. If it’s failed electrically or is stuck closed, the ignitor will glow but no gas reaches it — no flame, no heat. Gas valves can fail from age, electrical surges, or internal corrosion. This requires testing with a multimeter to confirm.
📟 Control Board Failure
The control board runs the entire ignition sequence. If it’s failed, it may power the blower but skip the ignition cycle entirely. Look at the board’s diagnostic LED — most boards have a blinking light that tells you the error code. Check the wiring diagram on the furnace door for what the blink pattern means.
✅ Check First: Thermostat Setting
Make sure the thermostat is set to HEAT (not COOL or OFF) and the fan setting is on AUTO (not ON). If the fan is set to ON, the blower runs continuously even when the furnace isn’t heating — this pushes room-temperature air that feels cold. Switch to AUTO and see if the problem goes away.
Quick Diagnostic Checklist
Before you replace parts, check these:
☑️ Thermostat set to HEAT and fan on AUTO?
☑️ Air filter clean?
☑️ Gas supply on? (Check the shut-off valve near the furnace)
☑️ Burners lighting at all? (Look through the inspection window)
☑️ Control board LED blinking a code?
☑️ Burners light then go out after a few seconds? (Flame sensor issue)
⚠️ Safety Note
If you smell gas at any point, leave the house immediately and call your gas company. Do not flip switches, use phones, or create any sparks near the furnace. Gas leaks are an emergency. For non-emergency electrical work, always turn off the furnace at the breaker and the gas shut-off valve before inspecting parts.
Need the Right Part?
Bring your old part to our Dallas store — we’ll test it for free and find the right replacement.
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Address: 10226 Plano Rd #104, Dallas, TX 75238
Phone: (214) 340-9421