A furnace that will not ignite is the most common emergency service call in the heating season. The thermostat calls for heat, you hear the blower start, but the burner either never lights or lights briefly and goes out. No flame means no heat.
The cause is almost always one of seven components. Here is how to diagnose each one.
Jump to Fix
- Bad Hot Surface Igniter
- Dirty or Faulty Flame Sensor
- Gas Valve Not Opening
- Faulty Pressure Switch
- Thermostat Wiring or Signal Issues
- Control Board Failure
- Blocked Condensate Drain Line
1. Bad Hot Surface Igniter
The hot surface igniter (HSI) is the most common cause of a furnace not igniting. It is a silicon carbide or silicon nitride element that glows red hot to light the gas. After thousands of heating cycles, the element becomes brittle and eventually cracks or burns out.
How to diagnose it:
- Remove the burner access panel and watch during a call for heat
- The igniter should glow bright orange within 30-60 seconds after the inducer motor starts
- If it does not glow at all, it is either burned out or not receiving power
- Look for visible cracks or breaks in the ceramic element
The fix: A bad igniter must be replaced. Turn off power to the furnace. Remove the retaining screw or clip holding the igniter in place. Gently pull the old igniter out. Handle the new one by the ceramic base only (skin oil on the element shortens its life). Insert and secure it, then restore power.
Important: Do not touch the glowing surface of the igniter with anything metal. If it sparks or arcs to ground, the igniter is grounded and needs replacement.
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2. Dirty or Faulty Flame Sensor
The flame sensor is a small metal rod mounted in front of the burner. It verifies that a flame is present so the gas valve stays open. When carbon buildup coats the rod, the sensor cannot detect the flame reliably and the control board closes the gas valve after 3-5 seconds.
How to diagnose it:
- Watch the burner through the sight glass
- If the flame lights for 3-10 seconds and then goes out, the flame sensor is almost certainly the cause
- The blower continues to run but no heat is produced
The fix: Turn off power and gas to the furnace. Locate the flame sensor (one screw, near the burner). Remove it and clean the metal rod with fine grit sandpaper or steel wool. Wipe it clean and reinstall it. This resolves the issue in 90% of cases.
If cleaning does not work, the sensor itself may be worn out or the ceramic insulator may be cracked. Replace it.
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3. Gas Valve Not Opening
The gas valve is an electrically operated solenoid valve. When the control board sends power, the valve opens and gas flows to the burner. If the valve fails to open, no gas reaches the igniter and no flame appears.
How to diagnose it:
- During a call for heat, listen for a faint clicking sound from the gas valve area
- If you hear the click but no flame appears and you smell a brief gas odor before the system shuts down, the valve may be open but the orifices are clogged
- If you do not hear the click, the valve is not receiving power from the control board
- Check the voltage at the valve terminals with a multimeter. You should see 24VAC during the ignition sequence
Common causes:
- A stuck valve (older units)
- A burned-out solenoid coil
- A clogged gas orifice (dust, rust, or debris in the gas line)
- The gas supply is shut off at the meter or a secondary shutoff valve
Check the gas supply first: Find the shutoff valve on the gas line near the furnace and confirm it is fully open. If the valve is half open, the gas pressure may be too low for the burner to light.
Safety note: Do not try to open or repair a gas valve yourself unless you are qualified. Gas leaks are extremely dangerous. If you suspect the valve is faulty, turn off the gas at the main shutoff and call a technician.
4. Faulty Pressure Switch
The pressure switch is a safety device that confirms the inducer motor is running and creating proper draft through the heat exchanger. If the switch does not close, the control board will not allow the igniter to glow or the gas valve to open.
How to diagnose it:
- Listen for the inducer motor (a small fan motor that runs before the burner ignites). If it runs but the burner does not try to light, the pressure switch may not be closing
- Check for a clogged condensate drain line (the most common reason the pressure switch fails to close)
- Use a multimeter to check if the pressure switch contacts are closed during inducer operation
- If the inducer motor is weak or the vent pipe is partially blocked, the switch may not get enough pressure
The fix: If the pressure switch is not closing, check the condensate drain first. Clear any blockage in the drain line. If the drain is clear, check the vent pipe for obstructions. If both are clear, the pressure switch itself may be faulty.
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5. Thermostat Wiring or Signal Issues
The thermostat tells the furnace when to start heating. If the wiring between the thermostat and the furnace is damaged, loose, or shorted, the heat signal may not reach the furnace. Low voltage thermostats are especially sensitive to wiring issues.
How to diagnose it:
- Check the furnace control board for an error code. Many boards flash diagnostic LEDs or display a code for “no call for heat”
- Check the thermostat display. If it is blank or showing “wait” or “recovering”, replace the batteries first
- Remove the thermostat faceplate and check for loose wiring at the W (heat) and R (power) terminals
- Check the wiring at the furnace control board. The thermostat wires connect to the low voltage terminal strip
The fix: Tighten any loose screws on the thermostat terminals. If the wires are broken or corroded, splice in new wire and reconnect. If the thermostat appears to work but no signal reaches the furnace, the thermostat itself may need replacement.
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6. Control Board Failure
The control board is the brain of the furnace. It receives the thermostat signal, activates the inducer, energizes the igniter, opens the gas valve, and monitors every safety switch. If the board fails, the ignition sequence stops at the very first step. The furnace does nothing at all.
Signs of a bad control board:
- No diagnostic LED is lit (meaning the board has no power)
- The board has visible burn marks, bulging capacitors, or corrosion
- The inducer motor runs but the igniter never gets power, even with a good igniter
- The board gives an error code you cannot clear
- The furnace responds to power cycling but fails again within a few hours
The fix: Control board replacement requires matching the board to your furnace model exactly. Most boards have a model number printed directly on them. Board replacement is a straightforward swap: label all wire connections, remove the old board, install the new one, and reconnect.
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7. Blocked Condensate Drain Line
High-efficiency condensing furnaces produce acidic water as a byproduct of combustion. This water drains through a plastic condensate line. If the line is clogged with algae, debris, or sludge, the water backs up into the heat exchanger. A float switch or pressure switch detects the backup and prevents the furnace from igniting as a safety measure.
How to diagnose it:
- Look for water pooling around the base of the furnace or dripping from the vent pipe
- Check the condensate drain line for visible blockages
- Locate the condensate trap (a small plastic assembly near the inducer motor). If it is full of water and not draining, it is likely clogged
- A gurgling sound from the furnace when the inducer runs can indicate water in the drain system
The fix: Turn off power to the furnace. Disconnect the condensate drain line from the furnace and flush it with water. Clean the condensate trap according to the furnace manual (usually a removable plastic fitting with a float). Reassemble and restore power.
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Parts You May Need
- Hot surface igniter
- Flame sensor
- Pressure switch
- Furnace control board
- Condensate drain tablets
- Digital multimeter
When to Call a Technician
Call a professional if:
- The gas valve needs replacement
- You suspect a cracked heat exchanger (this is dangerous and requires replacement)
- The control board replacement does not solve the problem
- The inducer motor or blower motor needs replacement
- You smell gas persistently
- You have checked all 7 causes and the furnace still will not ignite
A furnace that will not ignite in freezing weather is an emergency. If you have exhausted the basic diagnostics above, do not wait days to call. Overnight freezing can burst pipes and cause thousands in damage.