Furnace won't heat?

The blower is usually running but the air stays cool, and the cause is often a dirty filter, a thermostat setting, or an ignition problem. The first two are quick to check; ignition and gas parts are for a pro.

Not sure which one it is? Snap your furnace and we’ll narrow it down.

Is the furnace completely dead, with no fan or noise at all?

That's a power or control problem rather than a heating one. Furnace won't turn on

Likely causes

  1. Clogged air filter

    Most commonEasy DIY

    Weak airflow from the vents, a filter caked with dust, or the furnace shutting off after a few minutes on its high-limit switch.

    Part: $10–40 (filter)

  2. Thermostat settings or dead batteries

    Most commonEasy DIY

    Set to Cool or Off, a blank or dim display, a set temperature below the room temperature, or the fan set to On instead of Auto, which blows room-temperature air between heating cycles.

    Part: $5–15 (batteries)

  3. Dirty flame sensor or failed igniter

    CommonCall a pro

    The furnace starts, then shuts off within seconds, or clicks and glows but never lights. A flashing status light usually reports it.

    Part: $150–500 (installed)

  4. Pilot light out (older furnaces)

    Less commonCall a pro

    No small blue flame at the pilot window on a furnace that has a standing pilot.

  5. Failed gas valve, blower motor or control board

    Less commonCall a pro

    Error codes that repeat after a reset, a blower that hums but doesn't turn, or no gas reaching the burner.

    Part: $200–2,000 (blower motor at the top end)

Quick checks

If you smell gas (rotten eggs), or a carbon monoxide alarm sounds, leave the house immediately, then call your gas utility or 911 from outside. Don't flip switches or relight anything. Never open the burner compartment or touch the gas valve; that work is for a licensed technician. Everything on this page you can do yourself is a check: filter, thermostat, breaker and power switch.

  1. Set the thermostat to Heat and a few degrees above room temperature, and replace its batteries if the screen is dim or blank.
  2. Check the air filter and replace it if it's grey and clogged.
  3. Make sure the furnace power switch (it looks like a light switch, near the unit) is on and the breaker hasn't tripped.
  4. Open every supply vent and clear anything blocking them.
  5. Note any flashing status light or error code on the furnace door panel. It tells a pro what failed.
Sharpen the read

Diagnose your furnace in seconds, for free

No account, no phone number. Fixology’s AI reads your photo and tells you what’s likely wrong, how worried to be, and whether it’s a DIY fix.

Snap one of these:

  • the status light through the small window on the lower door, and its blink pattern
  • the model label on the side of the unit
  • the thermostat display

Add a note or more photos next. Nobody calls you.

When to call a pro

The furnace still won't heat after the filter, thermostat and power checks, it cycles on and off, an error light repeats, or there's any gas smell, soot, or a yellow burner flame. For lifespan and upkeep, see the furnace guide.

Typical cost: $130–500 for most repairs including the service call (about $320 on average). A diagnostic visit typically runs $50–150, often credited if you go ahead. A blower motor can reach $2,000.

Sources: HomeAdvisor: furnace repair cost (updated June 2026)

After a scan, Fixology can also help you find pros near you.

Related problems

  • Furnace won't turn on?

    Furnace

    Nothing happens when heat is called for, which is usually a tripped breaker, an off power switch, or a thermostat problem.

  • Heat pump not heating?

    Heat pumpFall

    The heat pump runs but the house stays cold, which is usually thermostat settings, a clogged filter, or an iced-over outdoor unit.

  • Washer won't drain?

    Washer

    Water is left in the drum after the cycle, which is usually a clogged pump filter or a kinked drain hose.

  • Fridge not cooling?

    Fridge

    The fridge section is warm or warming. The cause depends on whether the freezer is still cold, and the first fix is often free.

Fixology helps you understand your home — it isn't a substitute for a licensed pro on serious gas, electrical, or structural work.