A gas furnace won’t ignite when safety controls, ignition parts, gas supply, or airflow faults block the burner from lighting.
What “No Ignition” Looks And Sounds Like
You set heat. The blower or inducer starts. Clicks follow. Maybe the igniter glows, maybe not. The burner never lights, then the cycle stops. That pattern points to a lockout. The board tried and gave up. Good news: many causes are simple.
Safety First For A Non-Starting Gas Furnace
Smell gas or hear hissing? Leave the home, call your gas utility from outside, and wait. Treat carbon monoxide with care too. Use a working alarm and keep vents clear. See the U.S. Consumer Product Safety Commission guidance on home heating equipment and CO. For a plain-English primer on furnace types, venting, and AFUE terms, see the U.S. Department of Energy’s Energy Saver page on furnaces and boilers.
Quick Symptoms And Likely Causes
| Symptom | What It Suggests | First Thing To Try |
|---|---|---|
| Thermostat calls, no response | No power, bad batteries, loose wiring | Replace batteries, check breaker and furnace switch |
| Inducer runs, no glow or spark | Igniter failure, door switch open, control board issue | Shut power, reseat door, inspect igniter |
| Igniter glows, no flame | Gas valve closed, no gas, pressure switch or valve fault | Confirm gas cock position; try a restart cycle |
| Pilot lights then dies | Dirty flame sensor or weak thermocouple | Clean sensor or test thermocouple |
| Starts then shuts down fast | Dirty flame sensor, clogged filter, vent issue | Clean sensor, replace filter, check vent cap |
| Gurgle or drain splash | Condensate blockage on condensing units | Clear trap and hoses |
Gas Furnace Not Igniting: Fast Checks To Try
Thermostat And Power
Set the mode to Heat and the fan to Auto. Raise the setpoint several degrees. If it’s a battery thermostat, install fresh cells. At the furnace, flip the service switch off and back on. Check the breaker. Some units also have a fused control board; a blown low-voltage fuse stops a call for heat.
Door Switch And Control Panel
The blower door holds a safety switch. If the panel isn’t fully seated, the furnace stays dark. Reseat the cover until the latch clicks. While you’re there, find the sight glass. A blinking LED stores fault codes. Count the blinks, then match them to the code chart on the access door sticker.
Filter And Airflow
A matted filter chokes air. That triggers a high-limit switch and stops ignition or kills the flame fast. Slide the filter out and read the size. If it’s gray or bowed, replace it. Point the arrow toward the blower. If you use high-MERV filters, change them often. Closed supply or return grilles can create the same issue, so open them fully.
Pilot, Igniter, Or Spark
Standing Pilot Systems
Some older units use a constant pilot with a thermocouple. If the pilot is out, follow the lighting label inside the door. Turn the gas knob to Pilot, hold the button, light the pilot, and keep holding for 30–60 seconds before turning to On. If the flame won’t stay, the thermocouple may be weak or the pilot orifice may be dirty.
Hot Surface Igniters
Most modern furnaces use a hot surface igniter. It should glow bright orange. If it stays dark or looks cracked, it’s likely failed. Do not touch the element; skin oils shorten life. Many are simple to swap with the power off, a small socket, and the exact part.
Spark Ignition
Spark systems click rapidly at light-off. No spark points to a bad igniter, loose ground, or a control issue. Verify the burner area is dry and clean, then check connections.
Gas Supply And Shutoff Valve
Every furnace has a manual gas cock. The handle should be in line with the pipe for open. If the meter was off or the tank just filled, there may be air in the line. It can take several tries for the board to clear it. Run one restart per five minutes to avoid a hard lockout.
Flame Sensor Cleaning
The flame sensor confirms that burners lit. A light coating blocks the signal and the board shuts gas. With power off, remove the single screw, pull the rod, and clean the metal tip with fine emery cloth. Wipe with a dry cloth, reinstall, and run a heat cycle. If it fails again, inspect the sensor wire for damage.
Condensate And Pressure Switch
High-efficiency units produce water. A blocked trap or hose trips the pressure switch and kills ignition. Unplug the tube from the switch and the port on the inducer. Clear the trap, rinse the hoses, and make sure the drain slopes. Reconnect firmly. If water returns or the switch chatters, the vent or intake may be restricted.
Inducer, Vent, And Intake
The inducer clears exhaust before each light-off. If it’s noisy or slow, draft may be low. Outside, check the termination caps. Birds, leaves, and snow love those spots. Inside, confirm the intake isn’t blocked by boxes or laundry. A clear path restores draft and gives the pressure switch a clean signal.
Resetting Your Gas Furnace The Right Way
Many boards recover after a gentle reset. Turn off the thermostat. Cut power at the switch or breaker for 30 seconds. Restore power, wait a full minute, then call for heat again. If the unit tries to light three times and locks out, stop cycling it. Re-check the basics above before the next attempt. If you changed several things at once, put them back, then change one item per test; that simple A/B approach prevents rabbit holes and helps the control board reveal a clearer fault pattern.
When It Clicks But Never Lights
Clicks mean the control is stepping through the sequence. Match the step that fails to a likely part. If the igniter glows yet there’s no flame, check the gas valve, line pressure, or the pressure switch proving draft. If there is no glow and only clicks, the igniter, an open rollout, or the board could be stopping the cycle. If flames start then drop, the flame sensor, filter, or venting usually wins the blame.
Deeper Causes And Simple Fixes
Limit And Rollout Switches
Rollout switches guard against flames leaving the burner area. If one trips, find why before reset. Soot, rust flakes, or a blocked heat exchanger path can be a clue. Limits open on heat rise. A dirty coil, closed grilles, or a filter can push temps too high and stop ignition on the next call.
Control Board And Wiring
Loose low-voltage wiring causes random lockouts. Press gently on plug headers. Inspect for scorched spots on the board. Smell of burnt electronics? That points to a failing relay or trace. Label each connector before removal if a board swap is needed.
Combustion Air Supply
Open-burner furnaces draw air from the room. Tight houses and closed doors can starve them. Cracked windows near the unit are not a plan; restore a code-approved air path. Sealed-combustion units pull air from outside through a pipe. If that intake clogs, ignition stalls. Clear both intake and exhaust and keep kids’ gear, paint, and boxes away from the furnace cabinet.
LP Tank Or Meter Issues
LP tanks can run low. When service refills, pressure can drift until the regulator settles. Natural gas meters can be shut for work on the street. If neighbors had gas service work and your furnace quit right after, ask the utility to check the meter and relight pilots as required.
Sequence Cheat Sheet
| Step | What You Should See | If Missing, Check |
|---|---|---|
| Call for heat | Thermostat sends W signal | Thermostat, low-voltage fuse, wiring |
| Inducer starts | Steady motor sound | Inducer, pressure switch tubing, vent |
| Pressure switch proves | Click or code advance | Blocked drain, vent, intake, bad switch |
| Igniter heats or sparks | Orange glow or rapid tick | Igniter element, ground, board output |
| Gas valve opens | Soft whoosh | Gas cock, valve coil, line pressure |
| Flame proven | Burners stay lit | Flame sensor, filter, vent draft |
Costs, Parts, And When To Call A Pro
Igniters and flame sensors tend to be low-cost parts and quick swaps. Gas valves, control boards, and inducer motors sit higher on the scale and need testing tools. If you’ve run the basics and the unit still refuses to light, photos of the label, the blink code, and the vent piping help the technician arrive ready. That cuts return trips and gets heat back faster. Ask about warranty status and coverage before replacements.
Keep Ignition Reliable
Replace the filter on a schedule. Clear leaves from the vent cap before the heating season. Pour warm water and a little vinegar through the condensate trap each fall. Schedule a checkup before cold weather and request a written checklist of work performed. Ask for a combustion check, drain cleaning, and a reading of the flame signal in microamps.
Ignition Troubleshooting Checklist (Printable-Friendly)
1) Thermostat on Heat, setpoint above room, fresh batteries if used.
2) Furnace switch on, breaker on, door firmly latched.
3) New filter installed with airflow arrow toward blower.
4) Vent and intake clear outside; drain trap and hoses rinsed.
5) Igniter checked for glow or cracks; no touching the element.
6) Flame sensor cleaned with fine abrasive, dry cloth finish.
7) Gas cock inline with pipe; only one restart every five minutes.
8) Watch the blink code and note the sequence step that fails.
9) Stop and call a licensed HVAC company if you smell gas, trip a safety, or the unit locks out after repeats.