If your furnace is running but blowing cold air, the blower motor isn’t the problem. The heat exchanger isn’t getting hot in the first place. That usually means combustion failed, or a safety switch shut the burners down before you noticed anything was wrong.
Here’s what a technician checks, in order of likelihood.
Flame Sensor (Most Common)
The flame sensor is a small metal rod that sits in the burner flame. It sends a tiny electrical signal to the control board confirming combustion is happening. When it gets coated with oxidation or residue (which happens over years of normal use), the signal weakens and the board thinks there’s no flame. It shuts the gas valve off as a safety measure. The blower keeps running because it was already on.
A dirty flame sensor is probably the most common reason for a furnace that fires, runs for a few seconds, then blows cold. You may notice the burner lights, then goes out, then the whole thing tries to restart a couple more times before locking out.
Diagnosis: the tech pulls the sensor, looks for a white or gray film on the rod, and measures microamp output with a multimeter. Clean readings are typically in the 2-10 microamp range, depending on the system. Below about 2 microamps and the board won’t reliably confirm flame.
Fix is cleaning or replacing the sensor. It’s a quick call for a tech, and not one to guess at — the sensor rod is easy to bend or reinstall slightly loose, which recreates the exact same symptom. Better to have it done right once.
Hot Surface Igniter
Older furnaces used a standing pilot light. Most systems made in the last 20+ years use a hot surface igniter, a small ceramic element that glows orange-hot to light the burners. They’re fragile and they do fail. If the igniter cracks or burns out, the gas never lights.
Signs: you hear the furnace start, hear the gas valve click open, but never hear the whomp of ignition. Sometimes you can see the igniter glow cycle if you know where to look. Often you can’t.
A tech checks continuity across the igniter with a multimeter. An open circuit means it’s failed. Replacement is a pro job. The igniter is ceramic, has to be handled without bare hands (skin oils create hot spots that cause premature cracking), and the part has to match the system. A tech brings the right component and handles it correctly.
Limit Switch
The high-limit switch is a thermal safety device. When the heat exchanger gets too hot (usually from restricted airflow), the limit switch opens and cuts the burners. The blower stays on to cool things down, which is exactly what you feel: air moving, no heat.
A tripped limit switch almost always has a secondary cause: dirty air filter, blocked return vents, or a failing blower motor that isn’t moving enough air. If the filter is visibly clogged, that’s the first thing to swap. If a new filter doesn’t solve it and the limit keeps tripping, there’s something else restricting airflow or the switch itself is failing.
A tech checks the switch with a multimeter, but they also look at static pressure and filter condition to figure out why it tripped. Replacing the switch without fixing the root cause just means it trips again.
Gas Valve
If combustion isn’t happening and the igniter checks out fine, the gas valve itself might not be opening. This is less common, but it does happen. Gas valves can fail electrically (the solenoid coil opens) or mechanically.
A tech checks for 24V AC at the valve terminals during a call for heat. If voltage is present and the valve doesn’t open, the valve is the problem. If voltage isn’t present, the issue is upstream, usually the control board or a wiring fault.
Gas valve work stays with a licensed tech. Any time you’re touching gas-carrying components, there’s no margin for error.
Control Board
If all the components above test good, the control board is next. It’s the brain of the system. A failed board might not sequence the ignition correctly, might not send voltage to the gas valve, or might be locked out from repeated ignition failures.
Many boards have LED indicator lights that flash fault codes. The sequence of flashes corresponds to a code in the furnace manual (usually taped inside the panel door). That code tells the tech what the board thinks went wrong, which is a useful starting point, not always the final diagnosis.
What’s Safe to Check Yourself
A few things are worth doing before you call:
- Check the filter. A completely clogged filter can cause a limit switch trip. Take it out and hold it up to light. If you can’t see through it, replace it. Standard 1-inch filters should be changed every 1-3 months depending on the household.
- Check that the gas supply is on. Confirm the shutoff valve near the furnace is open (handle parallel to the pipe means open).
- Check the thermostat. Make sure it’s set to HEAT mode, not COOL or FAN ONLY, and that the set temperature is actually above current room temperature.
- Reset the furnace once. Some furnaces have a reset button (often red) on or near the burner assembly. Check your owner’s manual. Others reset by cycling power at the breaker or disconnect switch. One reset is fine. If it locks out again right away, stop there and call.
That’s about the limit of what’s safe to do without opening the cabinet and getting into 120V and 24V wiring, gas lines, and components where one wrong move creates a bigger problem.
Call a Tech
If the filter is clean, the gas is on, and the furnace still can’t produce heat after a reset, you need a tech. Flame sensor cleanings and igniter swaps are fast calls if that’s all it is. Limit switch diagnosis takes more time because you need to find the airflow cause, not just swap the part. Gas valve and control board work should always be done by someone licensed.
Furnaces 15-plus years old that need a board or heat exchanger repair are worth an honest conversation about. Sometimes the repair cost makes a replacement pencil out better. We’ll tell you that straight, even when it means a smaller ticket.
If you’re in the Tri-Valley or East Bay, Adrium Service handles furnace diagnosis and repair. Book online at adriumservice.com or call us directly. We’ll get you on the schedule fast, often same or next day when we can, and give you a clear answer on what it needs.