Furnace Not Heating? Here's Where to Start

Before You Touch Anything

The fastest technicians check the easy stuff first. These steps take 60 seconds and eliminate the most common causes:

1. 1
   Thermostat check

   Confirm it's set to HEAT (not COOL or OFF), fan is set to AUTO (not ON), and the setpoint is above the current room temperature. If it's battery-powered, swap the batteries.
2. 2
   Air filter

   A severely restricted filter is the single most common cause of furnace problems. A clogged filter triggers the high-limit switch, which shuts down the burners to prevent overheating. Replace if dirty and test again before going further.
3. 3
   Error codes

   Look at the LED on the control board before you do anything else. Count the flash pattern — most boards display a short flash code that points directly at the fault. The code key is usually on the inside of the furnace door.
4. 4
   Disconnect and power

   Check that the service disconnect is on. Check the breaker. Some furnaces have a door safety switch that cuts power if the panel is ajar.

Ignition System

Modern gas furnaces use either a hot surface igniter (HSI) or a spark igniter. If the furnace tries to start but doesn't ignite, this is where to look.

Hot surface igniters glow orange-red when working correctly. They're fragile — never touch the element with bare hands (oils from skin cause hotspots and premature failure). Inspect for cracks or breaks. Test resistance with a multimeter: most HSI elements read 40–90 ohms when cold, but always check the OEM specification.

Spark igniters use a spark to light a pilot flame, which then lights the main burner. Check the spark gap and look for carbon buildup that could bridge the electrode.

The rollout switch is a safety that trips if flames are rolling out of the burner box — a sign of heat exchanger issues or blocked flue. If it's tripped, find out why before resetting it.

Flame Sensor

This is the most common no-heat call cause after dirty filters. The flame sensor confirms to the control board that the burner actually lit. If it can't sense flame, the board shuts off the gas valve as a safety measure.

The symptom is unmistakable: the furnace tries to start, burners light for 2–3 seconds, then shut off. It tries again. After 3 attempts it goes into lockout mode.

The fix is almost always cleaning. Remove the sensor rod (one screw, one wire), and lightly clean the rod with fine steel wool or emery cloth until it's shiny. Don't use sandpaper — it leaves particles that insulate.

To verify after cleaning: with a multimeter set to DC microamps in series with the sensor wire, you should see >1.0 µA when the flame is established. Below 0.5 µA and the board may not recognize flame.

Pressure Switch

The pressure switch confirms that the inducer motor is creating enough negative pressure to safely vent combustion gases. If it doesn't close, the ignition sequence won't start.

Start by confirming the inducer motor is running — you should hear it spin up a few seconds before ignition. If it's not running, the inducer motor or its capacitor has failed.

If the inducer runs but the pressure switch won't close, check the condensate drain and trap. Blocked condensate is the most common cause of pressure switch failures on high-efficiency furnaces. A waterlogged trap creates back-pressure that holds the switch open.

Quick test: disconnect the pressure hose from the switch and apply gentle suction with your mouth. You should hear/feel the switch click closed. If it doesn't click with direct suction, replace the switch.

Limit Switch

The high-limit switch opens (cuts the burners) when the heat exchanger temperature exceeds its rating — usually 140–200°F depending on the furnace. A tripped limit means the furnace was overheating.

The error code will usually point here. Common causes: dirty filter, blocked supply or return registers, failed blower motor or capacitor, closed dampers.

Some limit switches reset automatically when the furnace cools. Others require manual reset. But if you reset without fixing the root cause, it will trip again.

Test the limit switch with a multimeter: it should be closed (continuity) at room temperature. If it reads open when cool, replace it.

Control Board and Gas Valve

The control board is the brain — it sequences every step of the ignition cycle. If you've checked everything else and still have no heat, measure 24V AC at the gas valve terminals during the call for heat.

If 24V is present at the gas valve and no gas flows, the valve has failed. Replace it.

If there's no 24V at the gas valve when there should be, trace back through the board. Is the board sending the signal? Is there a fuse on the board that's blown?

Control board failures are real but overstated. Confirm every input and output before condemning a board — replacing a good board is an expensive mistake.