Home heater not working: furnace checks in the right order, from thermostat to filter, and when to call a technician

Furnace not turning on: the checks in order

Most US homes heat with a forced-air furnace that warms air and pushes it through ducts. In the UK and much of Europe the usual system is a boiler heating water for radiators, which fails in different ways; for radiators, see how to bleed a radiator.

1. 01

   Thermostat mode and setting

   Make sure it is set to Heat, not Cool, Off or Fan only, and raise the setting a few degrees above the room temperature. Check that any schedule or vacation hold is not overriding you.

2. 02

   Thermostat power

   A blank screen usually means flat batteries; ENERGY STAR suggests changing thermostat batteries once a year. Some thermostats take power from the furnace, so a blank screen can also mean the furnace has no power.

3. 03

   Furnace power switch

   Near the furnace there is usually a switch that looks like a light switch, often on the unit, on a nearby wall or at the top of the basement stairs. It is easy to switch off by mistake. Make sure it is ON.

4. 04

   Breaker or fuse

   Check the panel for a tripped breaker labeled furnace, heat or air handler. Reset it once (fully OFF, then ON). If it trips again, leave it off and call a technician; see our circuit breaker guide.

5. 05

   Front panel

   Most furnaces have a safety switch that keeps the unit off while the blower door is loose. Seat the panel firmly.

6. 06

   Filter

   Slide out the filter, near the blower compartment or behind a return grille. If you cannot see light through it, replace it with the same size. A badly clogged filter starves the furnace of air and can make it shut down to protect itself.

7. 07

   Registers and returns

   Open supply registers and clear furniture, rugs and curtains off both supply registers and the larger return grilles. MSU Extension points out that furniture over a register cuts the flow of heated air.

8. 08

   Gas or fuel supply

   Check that other gas appliances work and that the valve on the furnace gas pipe is open (handle in line with the pipe). On propane or oil, check the tank level.

9. 09

   High-efficiency extras

   On a condensing furnace (plastic vent pipes, a small drain tube), check the condensate line and the outdoor pipes as described below.

10. 10

    Wait and watch

    After any reset, give the furnace several minutes: the inducer fan starts, the burners light, then the blower follows.

What the symptom tells you

Furnace blowing cold air

First look at the thermostat fan setting. On ON, the blower runs all the time, including between heating cycles, so you feel cool air; AUTO runs the blower only while the furnace is heating. If the fan is on AUTO and the air is still cold, check the filter and the condensate drain, then read the status light. Cold air with the burners off usually means the furnace has locked itself out after a failed ignition or an overheat, which the status light code will show.

Furnace short cycling

Short cycling is when the furnace starts, runs briefly and shuts off over and over. A clogged filter and closed registers are the easiest causes to fix, because restricted airflow can let the furnace overheat and switch the burners off. The thermostat location matters too: ENERGY STAR advises keeping it away from registers, lamps, appliances, doorways, windows and direct sunlight, ideally on an interior wall. If those are fine, a technician should check the flame sensing, the high-limit safety and whether the furnace is sized correctly for the house.

Condensate drain and vent pipes on high-efficiency furnaces

ENERGY STAR explains that most furnaces that qualify are condensing furnaces: they extract so much heat that water condenses out of the exhaust, which happens with systems over 90 percent efficient. You can usually spot one by its white plastic vent pipes running outside and a small drain tube, sometimes with a condensate pump.

• Condensate drain: ENERGY STAR lists the condensate drain in its maintenance checklist because a plugged drain can cause water damage, affect indoor humidity and breed bacteria and mold. Many furnaces stop heating when the drain backs up. Check that the tube is not kinked, that the floor drain or pump is clear, and that the pump runs when its reservoir fills.
• Outdoor intake and exhaust pipes: after snow, ice, leaves or a nest, clear the area around the pipe ends outside, gently and without pushing anything into them.
• Drain lines in cold spaces: in unheated attics or crawl spaces, a frozen drain line can stop the furnace. A technician can insulate or reroute it.

Furnace blinking light: reading the status code

Most modern furnaces have a small status light on the control board, visible through a window in the lower panel. A steady light or a slow, regular flash usually means normal operation; a set number of blinks means a specific fault. Codes vary by manufacturer, so match them to the chart on the label inside the furnace door or in the owner's manual.

1. 01

   Read before you switch off

   Many boards clear the code when power is cut. Count the blinks and note the pattern first.

2. 02

   Take a photo

   Photograph the code chart and the rating plate (model and serial number). It saves time on the phone with a technician.

3. 03

   Act only on simple codes

   Codes that point to an open limit, a pressure switch or a blocked drain often trace back to the filter, the vent pipes or the condensate line, which you can check. Codes for ignition, flame sensing or the gas valve are technician jobs.

4. 04

   One reset only

   Cycling power to retry is fine once. Repeated lockouts mean the furnace has found a real fault.

Pilot light out on an older furnace

Older furnaces have a standing pilot: a small flame that burns all the time to light the burners. Newer ones use electronic ignition instead, so there is no pilot to relight. If you have an older unit and the pilot is out:

• Check for a gas smell first. If you smell gas, do not try to relight anything: leave and call from outside.
• Follow the lighting instructions printed on the label on the furnace exactly, including the waiting time that lets gas clear before relighting.
• If the pilot will not light, will not stay lit, or burns yellow and lazy rather than a crisp blue, turn the furnace off at the gas control and call a technician. The flame-sensing part or the gas supply needs professional attention.

What to leave to a licensed technician

ENERGY STAR's maintenance checklist shows why the inside of a furnace is professional territory: the technician checks gas or oil connections, gas pressure, burner combustion and the heat exchanger, because a dirty burner or a cracked heat exchanger can make the furnace run less safely, and leaking gas connections are a fire hazard. They also inspect the flue piping for rust, disconnections and signs of back drafting, tighten electrical connections and test the controls through a full start-up and shut-down.

Leave these to a technician:

• Anything involving the gas valve, burners, pilot assembly, igniter or flame sensor beyond reading the code.
• Vent pipe joints, the chimney connection and the draft inducer.
• Electrical work inside the furnace or the control board.
• A furnace that trips its breaker or keeps locking out after a reset.
• Soot, scorch marks, a burning smell or yellow flames.

CPSC urges a yearly professional inspection of fuel-burning heating systems, including furnaces, chimneys, flues and vents, and ENERGY STAR recommends booking the heating check in the fall, before contractors get busy. Ask for the vent and chimney check to be included.

Ask the technician what the status code meant, what failed, and whether they checked the heat exchanger, venting and combustion.

Carbon monoxide and gas safety with a furnace

Every gas, propane or oil furnace produces carbon monoxide inside the burner area, and safe operation depends on the venting carrying it outside. CPSC estimates that more than 200 people in the US die each year from unintentional, non-fire carbon monoxide poisoning linked to consumer products, and lists heating equipment among the top contributors.

• Fit battery-operated or battery backup CO alarms on every level and outside sleeping areas, as CPSC advises.
• Never use a gas range, oven or grill to heat the house while the furnace is down.
• If you use a portable space heater as a stopgap, keep it at least 3 feet from anything that can burn, as CPSC's checklist advises, and give it a wall outlet of its own rather than an extension cord.
• Keep the area around the furnace clear of stored items, paint and cleaning products.

Sources

Official heating and safety guidance, checked on 27 October 2026.

• ENERGY STAR · Maintenance checklist ↗Monthly filter checks, condensate drain, gas connections, burner combustion, heat exchanger, electrical connections and fall heating check-ups.
• ENERGY STAR · A guide to energy-efficient heating and cooling ↗Filter change at least every three months, flue piping inspection, thermostat placement and batteries, condensing furnaces above 90 percent efficiency.
• US CPSC · Home heating equipment ↗Yearly professional inspection of fuel-burning heating systems and CO deaths linked to heating equipment.
• US CPSC · Carbon monoxide information center ↗CO symptoms, what to do and where to fit CO alarms.
• Kansas Corporation Commission · Natural gas safety ↗What to do and what to avoid if you smell gas indoors.
• Montana State University Extension · Clean furnace filters save energy dollars ↗Where the filter sits, monthly checks in the heating season and keeping registers and returns clear.