Furnace Clicking but Not Starting? Causes and Fixes

A furnace that clicks but does not start is usually failing at one step of its ignition sequence: a cracked or weak igniter, a dirty flame sensor, a closed gas valve, a blocked vent that trips the pressure switch, or an open door switch. Check the thermostat, the gas valve and the filter, watch the startup, and call a technician if it will not light.

A furnace start is a series of steps, and each can make a click. Knowing the order helps you work out which click is normal and where yours stops.

What a normal furnace start sounds like

When the thermostat calls for heat, a modern gas furnace follows a set order:

1. A click at the thermostat or control board. The call for heat reaches the furnace.
2. The inducer fan starts. A small motor clears the exhaust path and creates draft. You hear a hum or whir.
3. The pressure switch clicks closed. It confirms the fan is moving air through the vent.
4. The igniter warms up. A hot surface igniter glows orange for 20 to 40 seconds. On spark-ignition models you hear a fast ticking.
5. The gas valve clicks open. The burners light.
6. The flame sensor confirms the flame. If it does not, the furnace shuts off the gas.
7. The blower starts after a short delay, usually 30 to 90 seconds.

If your furnace stops at any step, you get clicks and no heat.

What the clicking tells you

How to fix it

1. Check the thermostat. Set it to Heat and a temperature five degrees above the room. Replace the batteries. A blank screen needs a different fix, as in a blank thermostat.
2. Check the power. Look at the furnace switch, which looks like a light switch on or near the furnace, and the breaker in the panel.
3. Check the front panel. Furnaces have a door switch that cuts power if the panel is off. Make sure the panel is seated and the switch is pressed in.
4. Check the gas valve. Find the shutoff valve on the gas pipe near the furnace. The handle should line up with the pipe (parallel) when open. If you use propane, check the tank gauge.
5. Replace the air filter. A very dirty filter can trigger a safety shutdown.
6. Clear the outside vents. Snow, ice or a nest can block the intake and exhaust pipes, which keeps the pressure switch from closing.
7. Watch the start. With the panel closed (follow your manual for the window), look at the igniter. If it never glows, or glows and then cracks or goes dark, it may be bad.
8. Look at the LED. The control board has a small light that blinks a code. Write the pattern down. The chart is usually inside the panel door.
9. Try one reset. Turn the furnace off at the switch for about 30 seconds, then on. If it starts, note when it fails again.

The usual culprits, in order of likelihood

A cracked or weak hot surface igniter. It is fragile and fails with age. If the igniter never glows, or glows weakly, replacement is the fix. It is a common repair and usually not expensive in parts.

A dirty flame sensor. If the burners light and go out within seconds, clean the sensor with a fine abrasive pad. The steps are in a furnace that keeps shutting off.

A blocked vent or condensate drain. High-efficiency furnaces use plastic vent pipes and a drain. A blocked pipe or a full drain trap trips a pressure or condensate switch. Clear the outside openings. If the drain is blocked, it needs cleaning.

A closed gas valve or no gas. After maintenance, a valve may be left closed. A gas outage or an empty propane tank also stops ignition.

A failed pressure switch. The switch can stick or fail even when the vent is clear. Replacement is a technician job.

A bad gas valve or control board. These are less common and need a professional.

Pilot light furnaces

If you have an older furnace with a standing pilot, there is no igniter. Clicking may come from the thermostat or a relay. Check whether the pilot is lit. If it is out, see relighting a pilot and replacing a thermocouple for relighting and thermocouple steps.

Is it worth repairing?

An igniter or a flame sensor is a modest repair. A pressure switch is moderate. A control board or a gas valve costs more. If your furnace is 15 to 20 years old and needs more than one major part, compare the repair quote with the price of a new furnace. Ask the technician about its condition, especially the heat exchanger.

When to call a professional

Call a licensed HVAC technician if:

• The furnace locks out again after a reset.
• The igniter does not glow, or you see no flame and hear gas.
• You smell gas or see soot or yellow flames.
• A carbon monoxide alarm sounds. Leave and call 911.
• You have cleared the filter, vents and sensor and it still clicks.
• You are not comfortable working near gas.

What not to do

• Do not bypass a safety switch.
• Do not keep resetting a furnace that will not light.
• Do not use the oven or a gas stove for heat.
• Do not open the burner area while the furnace has power.

Bottom line

Clicking means the furnace is trying and failing at one step. Check power, thermostat, gas valve, filter and vents, then watch the start to see where it stops. An igniter or flame sensor causes most no-start cases. If your furnace runs but blows cool air instead, see a furnace that runs but gives no warmth. If it starts and stops, read why a furnace keeps shutting off.