Nothing ends a cozy evening faster than discovering your Furnace not lighting when the temperature drops. While a mid-winter breakdown is stressful, many ignition issues in 2026 can be solved with basic cleaning or part replacement. This guide ranks solutions from most likely to least likely to help you diagnose the issue efficiently.
5 Common Causes for Furnace Ignition Failure
1Dirty or Corroded Flame Sensor
The flame sensor is a thin metal rod that sits in the path of the burner flame. Over time, it develops a thin layer of carbon (oxidation). If the sensor can’t “feel” the electricity from the flame, it tells the control board to shut off the gas to prevent an explosion.
Symptoms
- The furnace ignites successfully for 2–5 seconds, then shuts off.
- The furnace tries to relight 3 times and then enters “lockout” mode.
Care Plan
- Shut off the power to the furnace.
- Locate the flame sensor (usually on the opposite side of the burner assembly from the igniter).
- Remove the single screw holding the sensor in place.
- Use a piece of light sandpaper or a Scotch-Brite pad to gently rub the metal rod until it is shiny.
- Reinstall the sensor and restore power.
Common Mistakes
- Using a heavy-duty file that gouges the metal, making it rust faster.
- Touching the metal rod with oily fingers after cleaning it.
2Failed Hot Surface Igniter (HSI)
Most 2026 furnaces use a Hot Surface Igniter (HSI), which glows orange-hot to light the gas. These are made of silicon carbide or nitride and are extremely fragile. Like a lightbulb filament, they eventually crack or burn out.
Symptoms
- You can hear the inducer motor (small fan) start, but you never see an orange glow from the burners.
- The igniter has a visible “white spot” or a physical crack.
Care Plan
- Turn off the power and gas.
- Locate the igniter near the first burner.
- Unplug the wire harness and carefully remove the mounting screw.
- Check for continuity with a multimeter; if the circuit is “open,” it’s dead.
- Install a new igniter, being extremely careful not to touch the gray element with your fingers.
Common Mistakes
- Touching the element with bare hands; skin oils create “hot spots” that cause the igniter to shatter.
- Dropping the new igniter, as they are as fragile as glass.
3Tripped Pressure Switch
The pressure switch is a safety device that ensures the exhaust fan (inducer) is pulling air correctly. If it detects a blockage in the vent pipe or a failed motor, it will prevent the furnace from even attempting to light.
Symptoms
- The inducer motor starts, but the ignition sequence never begins.
- You hear a clicking sound followed by the furnace blower running but blowing cold air.
Care Plan
- Check the plastic tubing connected to the switch for cracks or water clogs.
- Ensure the exterior exhaust vent isn’t blocked by snow, ice, or bird nests.
- Use a paperclip to clear the small “port” on the furnace where the tube connects.
- If the inducer motor is noisy or slow, it may not be creating enough suction to close the switch.
- Replace the switch if it fails to close despite clear vents and a strong motor.
Common Mistakes
- Blowing into the pressure switch with your mouth; the pressure from your lungs can rupture the internal diaphragm.
- Bypassing the switch with a jumper wire, which creates a carbon monoxide hazard.
4Clogged Condensate Drain (High-Efficiency Models)
High-efficiency (90%+) furnaces produce water as they burn gas. This water drains through plastic hoses. In 2026, many “no heat” calls are actually caused by a clogged drain line that trips a safety float or creates back-pressure.
Symptoms
- Water pooling inside the furnace cabinet.
- The furnace “gurgles” before failing to light.
Care Plan
- Locate the condensate trap (usually a white plastic box inside the unit).
- Remove the hoses and flush them with warm water.
- Use a wet/dry vac to suck out any debris from the main drain line.
- Ensure the drain line has a proper downward slope.
- Refill the trap with a little water to ensure the “seal” is maintained.
Common Mistakes
- Neglecting to clean the drain trap annually.
- Forgetting to plug the hoses back in tightly, leading to water damage on the control board.
5Faulty Gas Valve or Solenoid
The gas valve is the final gatekeeper. If the control board sends power but the valve doesn’t open, no fuel reaches the burners. This is less common but requires professional attention if the internal solenoid has failed.
Symptoms
- The igniter glows bright orange, but no flame appears.
- You hear a “click” from the valve but don’t smell even a faint scent of gas.
Care Plan
- Verify that the gas shut-off valve on the pipe outside the furnace is in the “On” position.
- Check if other gas appliances in the house (like a stove) are working.
- Use a multimeter to check for 24V at the gas valve terminals during the ignition sequence.
- If 24V is present but no gas flows, the valve is likely faulty.
- Replacing a gas valve is a job for a licensed pro to ensure no leaks occur.
Common Mistakes
- Tapping the gas valve with a hammer to “unstick” it; this is dangerous and can cause a permanent leak.
- Assuming the valve is bad when the real issue is a safety lockout from a different sensor.
