The issue “furnace won’t ignite then shuts off” is a common HVAC problem that interrupts home heating and raises safety concerns. This article explains the likely causes, step-by-step troubleshooting, safety precautions, repair options, and preventive maintenance to restore reliable operation. Follow the guidance carefully and call a qualified technician when necessary.
| Symptom | Likely Causes | Quick Action |
|---|---|---|
| Igniter glows or sparks but burners shut down | Flame sensor dirty, weak flame, or control board fault | Clean flame sensor, inspect gas pressure, call pro |
| No ignition attempts | No gas supply, faulty thermostat, ignition control failure | Check thermostat, gas valve, pilot/igniter status |
| Ignites briefly then shuts off repeatedly | Blocked vent, flame rollout, limit switch trip | Inspect venting and flame pattern, seek professional service |
How A Typical Gas Furnace Ignites And Why It Shuts Off
A modern gas furnace uses a thermostat call to start the ignition sequence, energizing an inducer motor and opening the gas valve. An igniter (hot surface or spark) lights the burners, and a flame sensor confirms stable combustion. If the flame sensor does not detect a steady flame, the control board shuts the system off for safety.
Key Components include the thermostat, transformer, inducer motor, pressure switch, gas valve, igniter/pilot, flame sensor, limit switches, venting, and the control board. Failure or dirty components along this path commonly explain the pattern “furnace won’t ignite then shuts off.”
Common Causes And Diagnostic Clues
Dirty Or Fouled Flame Sensor
A flame sensor detects the burner flame via a small metal rod. Carbon buildup prevents reliable sensing, causing the furnace to ignite then shut off after a few seconds.
Diagnostic Clues: Furnace runs for a short time (5–30 seconds) then shuts down with a repeating cycle. The furnace often relights and repeats.
Weak Ignition Or Intermittent Pilot
A failing hot surface igniter or intermittent pilot flame will cause delayed or poor ignition, and the control board will shut the furnace for safety when sensing no stable flame.
Diagnostic Clues: Visible sparking or delayed glowing on start, unusual ignition noise, or a weak burner flame.
Gas Supply Or Gas Valve Issues
Insufficient gas pressure, closed gas shutoff, or a failing gas valve can produce an ignition that cannot sustain combustion, triggering shutoff.
Diagnostic Clues: Smell of gas near the unit (take precautions), burners click but do not stay lit, or furnace goes through the cycle without maintaining flame.
Blocked Venting Or Exhaust Problems
Blocked exhaust/vent pipes or a clogged chimney can cause combustion gases to accumulate and trigger safety switches like the pressure switch or rollout sensor, causing shutdown after ignition.
Diagnostic Clues: Strong exhaust odors, visible soot, or the inducer motor running loudly followed by shutdown.
Pressure Switch, Limit Switch, Or Rollout Switch Tripping
These safety devices shut the furnace down when conditions are unsafe. A faulty switch or genuine safety condition (like restricted airflow) can cause the furnace to ignite then quickly turn off.
Diagnostic Clues: Error codes on the control board, or a visible tripped manual switch. HVAC technicians can test switch continuity and vacuum tubing for pressure switches.
Thermostat, Control Board, Or Electrical Faults
Temperature sensor errors, thermostat wiring issues, or a failing control board may cause erratic ignition sequences and shutdowns.
Diagnostic Clues: Irregular sequencing, no call for heat despite thermostat setting, or error LED flash patterns on the furnace control board.
Step-By-Step Homeowner Troubleshooting
Only perform non-invasive checks if comfortable and safe. Turn off power at the furnace panel before opening covers. If unsure, stop and call a pro.
- Check Thermostat Settings: Ensure thermostat is set to heat, desired temperature is above room temp, and batteries (if any) are fresh.
- Inspect Air Filter: A clogged filter restricts airflow, causing limit switches to trip. Replace if dirty.
- Look For Error Codes: Many furnaces flash an LED with a fault code. Note the pattern and consult the furnace manual or manufacturer website.
- Examine Venting: For direct-vent furnaces, check external vent termination for debris, nests, or ice blocking airflow.
- Check Gas Supply: Confirm the gas shutoff valve to the furnace is open. If there is a gas odor, leave the area and call the gas company immediately.
- Observe Ignition Sequence: Watch the furnace during startup (from a safe distance). Note whether the igniter glows/sparks and how long the flame stays lit before shutdown.
How To Clean A Flame Sensor (Basic Guide)
Cleaning a flame sensor often resolves the “ignites then shuts off” pattern. This is a straightforward homeowner task with basic tools.
- Turn furnace power off and gas valve off, then remove the furnace access panel.
- Locate the flame sensor: a thin metal rod positioned in front of the burners.
- Remove the sensor mounting screw and gently pull out the rod.
- Use fine-grit sandpaper, emery cloth, or steel wool to gently clean carbon deposits from the sensor tip. Do not file aggressively.
- Wipe with electrical contact cleaner or a lint-free cloth, re-install, restore power, and test the furnace.
If cleaning works, the furnace should run normally. If problems persist, further diagnosis is required.
When To Call A Professional HVAC Technician
Contact a licensed technician if the homeowner troubleshooting does not resolve the issue, or if any of the following apply:
- Persistent ignition followed by shutdown cycles after cleaning the flame sensor.
- Suspected gas leak, unusual gas odor, pilot issues, or failure to open gas valve.
- Repeated trips of safety switches (rollout, pressure, or limit switches).
- Faulty control board, igniter replacement, or complex electrical/electronic diagnostics.
- Concerns about venting, carbon monoxide risk, or older equipment showing multiple simultaneous faults.
Expected Repair Costs And Timeframe
Costs vary by region, furnace type, and parts required. Typical ranges for common fixes are:
| Repair | Estimated Cost | Typical Time |
|---|---|---|
| Flame Sensor Cleaning | $0–$120 (DIY or pro service) | 30–60 minutes |
| Igniter Replacement | $150–$400 | 1–2 hours |
| Gas Valve Replacement | $300–$900 | 1–3 hours |
| Control Board Replacement | $250–$900 | 1–3 hours |
| Venting/Chimney Repair | $200–$1,500+ | Varies |
Prices reflect typical U.S. rates and may change with market conditions. Always request written estimates and warranties.
Safety Precautions And Carbon Monoxide Awareness
A furnace that ignites then shuts off could indicate incomplete combustion and risk of carbon monoxide (CO) when venting is compromised. Homeowners should have a working CO alarm on each level and near sleeping areas.
If the CO alarm sounds or someone experiences headaches, dizziness, or nausea, leave the home immediately and call emergency services and the gas company.
Preventive Maintenance To Avoid Future Ignition Problems
- Annual Professional Tune-Up: A technician inspects ignition, combustion, gas pressure, heat exchanger, and safety controls.
- Regular Filter Changes: Replace filters every 1–3 months depending on type and household conditions.
- Keep Vents Clear: Ensure outdoor vent terminals are free of snow, debris, and nests.
- Schedule Combustion Analysis: For older systems, periodic efficiency testing prevents hidden combustion issues.
- Document Fault Codes: Keep records of error codes and service visits to track recurring problems.
Troubleshooting Checklist For Technicians
Professional diagnostics focus on verifying sequence-of-operation and testing components:
- Check thermostat voltage and wiring continuity.
- Verify inducer motor operation and pressure switch function.
- Inspect igniter resistance and replace if out of spec.
- Test flame sensor current and grounding; replace if necessary.
- Measure gas valve operation and gas pressure at manifold.
- Examine venting, heat exchanger integrity, and rollout conditions.
- Scan control board for fault codes and observe LED patterns.
Frequently Asked Questions (FAQs)
Why Does The Furnace Light Then Go Out After A Few Seconds?
Most often a dirty flame sensor or failed detection system causes the control board to interpret a loss of flame and shut down the gas valve for safety.
Can A Dirty Filter Cause The Ignition To Shut Down?
A dirty filter can restrict airflow, causing overheating and tripping limit switches that shut off the burners. It may contribute to ignition shutdowns indirectly.
Is It Safe To Keep Restarting A Furnace That Keeps Shutting Off?
Repeated restarts are not recommended without diagnosis. Frequent cycling can damage components and indicate unsafe conditions; contact a professional if the problem persists.
Will Replacing The Thermostat Fix Ignition Problems?
Rarely. Thermostat issues can prevent a call for heat, but ignition-then-shutoff cycles are typically combustion or safety-control related rather than thermostat faults.
Resources And Further Reading
Homeowners should consult furnace manufacturer manuals for model-specific LED codes and safety instructions. The U.S. Consumer Product Safety Commission (CPSC) and the Environmental Protection Agency (EPA) offer guidance on carbon monoxide safety and combustion appliance maintenance.
For immediate safety concerns, contact the local gas utility or an emergency HVAC service provider. Regular professional maintenance significantly reduces the chance of a furnace failing to ignite reliably.