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Furnace · triage

Furnace not turning on

The short answer

Check the thermostat (and its batteries), the furnace power switch, the breaker and the filter. Do not open burner panels or relight anything. A gas smell or a CO alarm changes everything.

Gas and carbon monoxideIf you smell gas, leave and call from outside. If a CO alarm sounds, get everyone into fresh air and call 911.

Know your shutoffs

Gas shutoffAt the gas meter outside (a valve on the pipe before the meter), and a smaller shutoff valve on the gas line next to each appliance.Electrical panel and main breakerThe breaker panel (or fuse box), often in a garage, basement, utility room or hallway.

Every outcome on this card

Emergency nowGas smell or CO alarm

Leave. Call from outside.

  1. Get everyone outside
  2. Call 911 or the gas utility from outside
  3. Stay out until responders say it is safe
WatchA thermostat issue

Fix the setting or batteries; give it 10 minutes.

  1. Replace thermostat batteries if the display is blank
  2. Set Heat, a few degrees above room temperature
  3. If nothing happens in 10 minutes, go back to the checks
WatchPower restored

Watch that it keeps running.

  1. Give it 10 minutes to start its cycle
  2. If the breaker trips again, leave it off and call
  3. Log it
This weekFilter or door

Fix it, then try again.

  1. Turn the furnace power switch off
  2. Replace a clogged filter and close the door panel firmly
  3. Switch on and wait 10 minutes If it starts then quits, call a technician.

Overheating from a clogged filter trips the high-limit safety; many furnaces then lock out.

TodayNo heat after the basic checks

Call an HVAC technician; today in cold weather.

  1. Note any blinking light code on the furnace’s control board window Many furnaces show a code through a small window; it helps the technician.
  2. Don’t relight or open burner panels
  3. Use safe heat meanwhile Never an oven, grill or generator indoors.

Common causes are a failed igniter or flame sensor, a pressure switch, the inducer motor or the control board.

Related

Furnace short-cycling (turns on and off quickly)Your carbon monoxide alarm is soundingFrozen pipes: no water, or only a trickle

Written from

Original casaq guidance, updated 2026-10-05. General guidance, not a diagnosis; licensed-pro review pending. Severity levels: Emergency now / Today / This week / This month / Watch. Send a correction.

Step 1

Do you smell gas, or is a CO alarm sounding?