If a gas furnace won’t turn on, verify thermostat settings, power, filter, safety switches, and gas supply before calling a licensed technician.
Safety First: When To Stop And Call
Your safety comes first. If you smell gas, hear a hiss, or your carbon monoxide alarm sounds, leave the home and call emergency services or your gas utility from outside. The CDC furnace safety fact sheet explains why carbon monoxide is dangerous and what to do. Never bypass doors, jump wires, or tape over safety switches. Cut power at the furnace switch or breaker before removing any panel. If anything seems unsafe, stop and bring in a pro.
Gas Furnace Not Turning On: Quick Checks
Most no-heat calls trace back to a short list of easy fixes. Run these quick checks first. They take minutes and often save a service visit.
| Checkpoint | What It Solves | How To Do It Safely |
|---|---|---|
| Thermostat mode | Heat call not sent | Set to HEAT, fan AUTO, and raise the setpoint 3–5°F. |
| Thermostat power | Dead batteries or loose face | Install fresh batteries; reseat the faceplate until it clicks. |
| Furnace switch | Unit has no power | Flip the nearby toggle to ON; it looks like a light switch. |
| Breaker | Tripped circuit | Find the furnace breaker, move fully OFF, then ON once. |
| Blower door | Door switch open | Seat the lower panel firmly so the switch is pressed. |
| Air filter | Overheat lockout | Slide out, check light through the media, replace if clogged. |
| Condensate line | Drain safety open | Empty the pump reservoir; clear kinks; confirm outlet power. |
| Gas shutoff | No fuel to valve | Handle parallel with pipe is ON; perpendicular is OFF. |
| Intake/exhaust | Pressure switch won’t close | Clear leaves, snow, or nests at outdoor pipe terminals. |
Understand The Startup Sequence
Knowing the order of operations helps you spot where things stop. A heat call should start the inducer fan, close the pressure switch, light the igniter, open the gas valve, and prove flame at the sensor. Then the main blower starts after a short delay. Watch and listen through the view window with panels secured. If the sequence stops at the same stage every time, you’ve found your clue.
Thermostat And Power Basics
Confirm the thermostat is on HEAT, not COOL or OFF. Many smart stats need a steady C-wire; if yours drops out between cycles, fresh batteries may keep it alive. If the display is blank, restore power at the furnace switch and breaker. Some control boards include a small automotive-style fuse; check only with power off. If it’s blown, another fault may be present, so match the original rating and watch for repeat trips. Loose low-voltage wires at the control terminals can also kill the heat call; gently tug each conductor to make sure it’s snug.
Door And Safety Switches
Most furnaces won’t run with the blower door loosened. Seat both upper and lower panels until the safety switch is fully pressed. If the control board has a sight glass, note any blink code after a failed start. Codes often point to pressure, ignition, or flame sense issues. Use the legend on the door sticker to translate the flashes before you cycle power.
Airflow, Filter, And Overheating
A clogged filter starves airflow. The heat exchanger overheats, the limit switch opens, and the board locks the burner until the furnace cools. Replace 1-inch filters every 1–3 months and thicker media on the manufacturer’s schedule. Make sure return grilles are open and supply registers aren’t shut. After a limit trip, the blower may run by itself; once cooled, a new cycle should start. If the limit opens again right away, leave the furnace off and call a pro to check the blower speed, ducts, and temperature rise.
Condensate And Pressure Switch Tips
High-efficiency furnaces produce water. A full condensate trap or a failed pump keeps the pressure switch from closing, so the board never allows ignition. With power off, empty the pump reservoir and replug it. Inspect vinyl tubing for sags filled with water and ensure the trap is seated. At the exterior, clear ice or debris at the PVC terminations. These quick steps resolve many no-start calls, especially after storms.
Ignition, Flame Sensor, And Lockouts
Hot surface igniters glow bright orange. If you never see a glow, the igniter could be cracked or the board isn’t sending power. If it glows and the burners light but shut down within seconds, the flame sensor may be dirty. With power off, remove the single screw, slide out the rod, and wipe gently with a fine abrasive pad. Reinstall firmly. If the board reached its retry limit, it may lock out until you cycle power at the switch. Repeated lockouts call for a technician to check grounding, gas pressure, and burner carryover.
Gas Supply And Valves
Verify the manual gas cock next to the furnace is ON. The handle in line with the pipe means open. If other gas appliances are also out, call your utility. Never loosen gas fittings or apply open flame. If you smell gas, evacuate and wait for help outside.
Blower Runs But No Heat
If the fan runs but air is cool, the burner stage isn’t completing. Check the filter, condensate safety, and intake blockage first. Confirm the thermostat isn’t set to FAN ON, which forces continuous blower. If the control board shows a limit or rollout code, stop and bring in a pro to inspect the heat exchanger and flue path.
Reading Blink Codes
The control board LED is your built-in narrator. Short and long flashes map to common faults printed on the door label. While codes vary by brand, patterns tend to cluster around pressure switch, ignition, flame sense, and limit trips. Note the code before cycling power; a restart clears the history and erases that clue.
| LED Pattern | Typical Meaning | Next Step |
|---|---|---|
| Steady on | Power to board | Heat call may be missing; recheck thermostat wiring. |
| Slow single blink | No flame sensed | Clean flame sensor; verify ground; retry once. |
| Two blinks | Pressure switch open | Clear intake/exhaust; check condensate pump. |
| Three blinks | Limit switch open | Replace filter; open vents; let unit cool. |
| Rapid flash | Polarity or grounding | Have an electrician correct wiring. |
Pilot, Igniter, And Sensor Basics
Standing-pilot furnaces are rare, but if you have one, relight only per the label and only if you never smell gas. Newer units use an electronic igniter and flame sensor. Igniters are fragile; handle the ceramic base, not the glowing element. For sensors, a light polish is all that’s needed. Heavy sanding can shorten life and cause nuisance dropouts.
Vents, Chimneys, And Freeze Ups
Sidewall vents can clog with frost after a storm. Rooftop terminations collect leaves and twigs. Any blockage alters pressure and stops the sequence. Clear what you can reach from the ground. If you must climb, wait for a pro. In extreme cold, a hair dryer at the outdoor elbow can melt light frost, but keep water away from electrical parts and stay clear of moving fans.
When The Thermostat Is The Culprit
Loose low-voltage wires, a miswired new stat, or a missing jumper between R and RC can block the heat call. If heat works by shorting R to W at the furnace terminals, the thermostat is likely at fault. Replace weak batteries twice a year and set a calendar reminder before heating season. If you upgraded to a smart stat, confirm the equipment type, fuel, and staging in its setup menu.
When You Need A Professional
Call a licensed technician for gas valve diagnostics, control board replacement, repeated lockouts, combustion checks, or any flue, rollout, or heat-exchanger concern. Annual service also catches worn igniters and dirty burners before winter. ENERGY STAR’s HVAC maintenance checklist shows what a thorough tune-up includes and why pre-season visits prevent surprise outages.
Prevent The Next No-Heat
Small habits keep heat steady. Change filters on schedule. Keep storage away from the furnace cabinet. Vacuum lint around the burner compartment with power off. Clear snow from outdoor terminations after storms. Test your carbon monoxide alarms every month and replace units at end of life per the label. Schedule a pre-season check each fall to catch weak parts before the first cold snap.
Quick DIY Decision Tree
Use this simple path to decide your next move.
- Thermostat set to HEAT and raised? If no, set it and wait two minutes.
- Furnace switch ON and breaker not tripped? If tripped, reset once.
- Door seated and filter clear? Reseat and swap as needed.
- Condensate lines clear and pump running? Empty and replug.
- Outdoor pipes clear? Remove snow or leaves.
- Still no heat? Cycle power, note any blink code, and share that code with your technician.
What A Normal Cycle Looks Like
Stand back and watch a healthy start: inducer runs, click from the pressure switch, igniter glows, burners light smoothly, steady blue flame, blower starts, then shuts down a minute after the thermostat is satisfied. Any stumble points you to the faulty stage. Take a short video of the sequence and the LED code; that clip helps a technician bring the right parts on the first trip.
Smart Moves During Cold Snaps
While you wait for service, keep pipes safe and temps stable. Open sink cabinets on exterior walls, let a pencil-thin stream run, and set the thermostat no lower than 60°F. Use only UL-listed electric space heaters on a dedicated circuit, placed three feet from anything that can burn. Never use ovens or grills for heat, and keep portable generators outdoors, away from doors and windows.