Furnace not turning on? Check these 6 things before you call anyone

Most "dead" furnaces aren't dead. Roughly half the no-heat calls an HVAC tech runs in January end with a flipped switch, a dirty filter, or a thermostat battery — and a $150 bill for the drive. Spend fifteen minutes on this list first. Then, if it's still cold, you'll know exactly what to tell the tech, which shortens the visit.

The six checks, in order

1. Thermostat. Set to HEAT, temperature at least 3°F above the room. If the screen is blank or dim, replace the batteries (two AAs, usually). If it's a smart thermostat, check it hasn't lost power or Wi-Fi and reverted to an "away" schedule.
2. The furnace switch. There's a light switch on or next to the furnace that looks exactly like a wall switch. It gets bumped off during laundry or by a kid. Flip it on.
3. The breaker. Even gas furnaces need 120V for the blower and the control board. Find the breaker labeled FURNACE or HVAC. If it's tripped (handle in the middle), push fully OFF, then ON. If it trips again immediately, stop — that's a short, and it's a tech's job.
4. The filter. Pull it out and hold it up to a light. If you can't see light through it, the furnace is likely shutting itself down on a high-limit switch to keep from overheating. Replace it ($10–$25), wait 30 minutes for the limit to reset, and try again. This is the single most common cause.
5. The furnace door. Every modern furnace has a door safety switch. If the blower compartment door isn't fully seated, nothing runs. Push it until it clicks.
6. The condensate drain (high-efficiency units). If you have a white PVC pipe coming off the furnace and a small pump or trap, a clogged drain trips a float switch and locks the furnace out. Clear the trap, pour a cup of water through to confirm it drains, and the furnace will usually restart.

Read the error code

If it still won't run, look through the small window on the lower furnace panel for a blinking LED. Count the blinks — the code chart is printed on the inside of the door. The codes most homeowners see:

| Code pattern (typical) | Meaning | Who fixes it | |---|---|---| | Steady on / no light | No power or dead board | You (power), tech (board $300–$600) | | Pressure switch open | Blocked flue, bad inducer motor, clogged condensate | You (condensate), tech ($250–$600) | | Ignition failure / flame not sensed | Dirty flame sensor | You ($0) — see below | | High limit open | Airflow — filter, closed vents, failing blower | You (filter), tech if blower ($400–$800) | | Rollout switch tripped | Flame escaping the burner box | Tech only. Do not reset repeatedly. |

The one real repair you can do: clean the flame sensor

A furnace that lights for 3–5 seconds and then shuts off, three times in a row, then locks out — that's a dirty flame sensor nine times out of ten. It's a thin metal rod sticking into the burner flame, held by one screw.

1. Kill power at the switch. Close the gas valve on the furnace line (handle perpendicular to the pipe).
2. Remove the one screw, slide the sensor out.
3. Rub the rod with a dollar bill or fine steel wool until it's shiny. Don't use sandpaper.
4. Reinstall, open the gas valve, restore power.

Fifteen minutes, $0. A tech charges $150–$250 for the same thing.

Where DIY stops

Anything past the flame sensor involves gas, combustion, or 120V control wiring, and that's where this becomes a hire.

Call a pro — don't attempt — if:

• The breaker trips again after one reset.
• The rollout switch has tripped. That means flame left the burner box. Resetting it to "see if it holds" is how house fires start.
• You smell gas. Leave the house, call the gas utility, then an HVAC company.
• The inducer or blower motor hums, squeals, or doesn't spin. Motors are $400–$900 installed, and the diagnosis needs a meter.
• The furnace is over 15 years old and the quote is over $800. Put that money toward replacement; see the AC repair-or-replace math — the same logic applies to furnaces.

Gas valve, burner, and heat exchanger work is never DIY. A cracked heat exchanger leaks carbon monoxide into the house, and the only way to find one is with the panel off and a combustion analyzer. That's a licensed technician with insurance, not a Saturday project.

What a fair service call looks like

Diagnostic visit: $90–$180, usually credited toward the repair. Common repairs installed: igniter $150–$300, flame sensor $100–$250, pressure switch $200–$400, control board $400–$700, inducer motor $500–$900. If you're quoted more than that, get a second bid — the Find a Pro page pulls two to four local HVAC quotes in a day, and the price spread between companies on the same repair is routinely 40%.