Furnace clicking but won't ignite

A furnace that won't light, or clicks without a flame, is sometimes a setting, power, a clogged filter or a closed gas handle. Repeated clicking with everything else checked usually means the igniter or flame sensor, which is a technician's job.

Also searched as: furnace won't ignite, furnace clicking but not turning on, furnace clicking but not igniting, furnace not kicking on.

Before you start. If you smell gas or rotten eggs, stop here. Leave the house, don't touch switches or use your phone inside, and call 911 or your gas utility once you are outside.

A hand sliding a new pleated air filter into a furnace filter slot

Every question and answer on this page, in order

Question 1: Do you smell gas or rotten eggs anywhere in the house?

Yes, or I'm not sure: Gas smell: leave the house now
No: go to question 2

Question 2: What happens when the thermostat calls for heat?

Set the thermostat a few degrees above room temperature and stand by the furnace for a couple of minutes.

Nothing at all, no sound from the furnace: go to question 3
Repeated clicking, but no flame and no heat: go to question 4
The thermostat isn't on Heat, or is set below room temperature: Likely cause: the thermostat isn't calling for heat

Question 3: Check the furnace's power. What do you find?

The power switch often looks like a light switch on or near the furnace. Look only; don't remove panels.

The switch was off, or the front door panel wasn't fully seated: Likely cause: the furnace switch or door panel
The furnace breaker was tripped: go to question 5
Switch on, door seated, breaker on: go to question 4

Question 4: Check the filter and the gas handle on the pipe into the furnace. Which is true?

HVAC.com explains that a gas handle perpendicular to the pipe is off; in line with the pipe is on.

The filter is gray and clogged: Likely cause: a clogged air filter
The gas handle sits across the pipe: Likely cause: the gas handle is off
Filter is clean and the handle lines up with the pipe: Likely cause: the igniter or flame sensor

Question 5: Reset the breaker once, firmly off then on. Did it hold?

Open only the hinged panel door, never the metal cover behind it. Reset it only once. HVAC.com says if it trips again, don't try to turn it on again.

It held: go to question 4
It tripped again: A breaker that keeps tripping: call an HVAC technician

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.