Furnace Won’t Stay Lit: Causes, Fixes, and When to Call a Pro

When a gas furnace won’t stay lit, comfort drops and energy costs can rise fast. This guide explains why a furnace flame goes out or the system short cycles, how to troubleshoot safely, and which fixes are DIY-friendly versus professional. Learn the most common causes—like a dirty flame sensor, clogged filter, pressure switch issues, or vent blockages—and how to prevent repeat failures so heat stays reliable all winter.

How A Gas Furnace Stays Lit

A modern forced-air gas furnace follows a precise start-up sequence. The thermostat calls for heat, the inducer fan starts, and the pressure switch confirms proper draft. The control board energizes the igniter, opens the gas valve, and the burners light. A flame sensor verifies flame, then the blower starts to distribute heat.

If any step fails, the furnace shuts down for safety. Older furnaces may have a standing pilot with a thermocouple; newer units use hot surface or spark ignition. Most “won’t stay lit” issues occur after ignition, when safety controls trip and stop gas flow.

Safety First: When To Stop And Call A Technician

• Smell gas or hear a persistent hiss: Leave the area and call the gas utility and 911 from outside.
• Carbon monoxide alarm sounding: Exit immediately and contact emergency services. Do not re-enter until cleared.
• Burner flame rolling out, soot, or scorched wiring: Possible heat exchanger or combustion issue—professional service required.
• Loud bangs on ignition, repeated tripping, or melted components: Turn off power and gas to the furnace and call a pro.
• Water pooling inside a high-efficiency furnace: Could indicate condensate backup affecting safety switches.

Quick Checks A Homeowner Can Do

Before deep troubleshooting, these basics often restore heat quickly:

• Thermostat: Set to Heat and a higher temperature; replace batteries if applicable. Verify “cycles per hour” or “compressor protection” isn’t limiting heat calls.
• Power: Confirm the furnace switch is on, the breaker isn’t tripped, and the blower door is secure (door switch engaged).
• Airflow: Replace a dirty filter, open supply registers, and clear return grilles.
• Gas supply: Ensure the gas valve at the furnace is parallel to the pipe (open). For LP, confirm tank level.
• Outside: For high-efficiency models, check intake/exhaust PVC pipes for ice, snow, leaves, or nests.
• Condensate: Empty a full condensate pump reservoir; ensure the drain line isn’t kinked or clogged.
• Error code: Look through the furnace’s sight glass for a blinking light; note the pattern for diagnostics.

Common Reasons A Furnace Won’t Stay Lit

Dirty Flame Sensor

The flame sensor confirms that burners are lit. When coated with oxide, it can’t detect flame, so the control board shuts the gas valve after a few seconds. Symptoms: burners ignite, run 5–10 seconds, then turn off repeatedly.

DIY fix: Cut power and gas. Remove the sensor (single screw), gently polish the metal rod with fine steel wool or emery cloth, wipe clean, and reinstall. Do not sand the porcelain. If cracked or pitted, replace the sensor.

If cleaning doesn’t help, wiring, grounding, gas pressure, or control board issues may be involved. A tech can measure microamp flame signal and correct underlying problems.

Clogged Filter Or Blocked Airflow (High Limit Trip)

Restricted airflow overheats the heat exchanger, causing the high limit switch to open and kill the burners. The blower may continue to cool the furnace, then burners relight and shut down again, creating short cycling.

DIY fix: Replace a dirty filter (1-inch filters often need monthly changes; check MERV rating per manufacturer). Open closed registers, clear return grilles, and check for blocked ducts. Excessively high static pressure, dirty blower wheels, or a clogged A-coil require professional cleaning or duct adjustments.

Persistent limit trips shorten furnace life; ensure filters fit snugly to prevent bypass dust that coats the exchanger and coil.

Pressure Switch Or Venting Problems

The pressure switch verifies the inducer is moving combustion gases. If venting is restricted, the switch won’t prove or drops out, shutting burners. High-efficiency furnaces are sensitive to vent slope, icing, and condensate drainage.

DIY checks: Inspect the intake and exhaust terminations for ice, snow, lint, leaves, or nesting. Clear obstructions carefully. Ensure PVC joints are intact and not sagging with water. Look for cracked or water-filled pressure switch tubing and reseat snugly.

If issues persist, a technician can test draft, measure pressure switch operation, verify proper vent sizing, and correct slope (often 1/4 inch per foot back toward the furnace on condensing models).

Igniter Or Pilot Problems

Hot surface igniters can crack or weaken, preventing reliable light-off. Spark systems can misalign. Standing pilots rely on a thermocouple or thermopile to hold the gas valve open; a weak sensor lets the pilot go out.

DIY options: For standing pilots, follow the lighting instructions on the panel. Clean a sooty thermocouple with fine steel wool; replace if it won’t hold. For hot surface igniters, visual cracks or white spots suggest failure; avoid touching the element with bare fingers.

Ignition timing, gas pressure, and burner alignment often need professional adjustment. If a pilot won’t stay lit after cleaning or relighting, call a technician.

Gas Supply Or Regulator Issues

Closed valves, low LP fuel, or a malfunctioning regulator can cause weak or intermittent flame. Utilities may also interrupt service for maintenance.

DIY checks: Verify the manual gas cock at the furnace is open. For LP, confirm tank level above 20%. If other gas appliances are affected, contact the gas supplier. Do not attempt regulator adjustments.

Professionals can measure manifold pressure, clock the meter, and verify correct orifice sizing for altitude and fuel type.

Flame Rollout Or Heat Exchanger Problems

Flame rollout indicates combustion gases aren’t flowing correctly—potentially due to a blocked heat exchanger or severe venting issue. Rollout or high limit switches will shut the furnace down for safety.

Warning: Soot, strong odors, warped burners, or scorch marks require immediate professional service. Heat exchanger cracks can leak carbon monoxide and must be repaired or the furnace replaced.

Control Board, Sensors, Or Wiring Faults

Corroded terminals, poor ground, or a failing control board can drop the flame signal and close the gas valve. Loose door switches can intermittently cut power, mimicking short cycling.

DIY checks: Ensure the blower door is firmly latched. Many issues require a multimeter and OEM diagnostics; a technician can read board fault codes, verify ground, and inspect harnesses for heat damage.

Blower Motor Or Capacitor Failure

If the blower fails to start or runs slowly, the heat exchanger overheats and the limit opens, shutting the burners. PSC motors often fail due to capacitors; ECM motors report fault codes.

DIY checks: Listen for a humming blower or intermittent start. Capacitors store charge and can be dangerous; replacement should be done by a professional who can also measure static pressure and airflow.

Oversized Furnace Or Tight Duct System

Short run times with frequent restarts can indicate an oversized furnace or a duct system with high static pressure. Rooms may heat rapidly near the thermostat while distant spaces remain cool.

Possible remedies: Adjust blower speed, open more registers, add return air, install a two-stage or modulating system during replacement, and optimize thermostat cycles-per-hour settings.

Thermostat Calibration Or Wiring

Incorrect cycles-per-hour, poor placement near a supply register, or weak batteries can cause frequent shutoffs. Loose connections at the furnace control can also drop the heat call.

DIY checks: Replace batteries, move the thermostat away from drafts or heat sources, and confirm wiring is secure at R and W terminals. For smart thermostats, ensure a proper C-wire or a compatible power extender kit.

Step-By-Step Troubleshooting Guide

1. Confirm the symptom: Note whether the burners light then go out, or fail to light at all. Listen for the inducer and watch the igniter.
2. Check power and gas: Furnace switch on, breaker set, blower door latched, gas valve open.
3. Replace or remove a clogged filter: If the filter is caved in or black with dust, install a clean one of the correct size.
4. Inspect intake/exhaust: Clear any snow, ice, or debris from PVC terminations. Ensure at least 12–24 inches of clearance above expected snowfall.
5. Read the error code: Record the blink pattern. Compare with the unit’s service label to target the circuit at fault.
6. Clean the flame sensor: Power off. Remove, polish gently, wipe, reinstall, and restore power.
7. Drain condensate: Empty a full pump, clear kinks in tubing, and ensure the trap is not dry or blocked.
8. Thermostat checks: Increase target temperature by 5°F, verify cycles-per-hour settings, and replace batteries.
9. Test run: Observe the full heat cycle for at least 10–15 minutes. If the flame still drops out, schedule service.

Decoding Furnace Error Codes

Manufacturers use unique blink codes, but common patterns often point to the same systems. Always verify with the wiring diagram or service chart on the inside panel.

Note: Code meanings vary by brand and model; the panel chart is the authority.

Special Notes For High-Efficiency (Condensing) Furnaces

Condensing furnaces extract extra heat, generating acidic condensate that must drain properly. Any blockage can trip the pressure switch or cause flame dropout.

• Condensate trap and pump: Clean annually. Algae or sludge can plug the trap. A failed pump switch will shut the furnace down.
• Vent slope: Intake and exhaust must be correctly sized and sloped to drain condensate back to the furnace or safely outside.
• Freezing risk: Low outdoor temperatures can ice PVC terminations. Keep clearances and use screens recommended by the manufacturer.
• Intake air: A blocked intake starves combustion and can extinguish the flame. Check both pipes, not just the exhaust.
• Neutralizer: If draining into sensitive plumbing, ensure the condensate neutralizer media is not exhausted.

Pilot Light Won’t Stay Lit (Older Furnaces)

Standing-pilot furnaces rely on a small flame to light the burners. If the pilot won’t stay lit, the thermocouple or thermopile may be weak, dirty, or mispositioned.

• Clean: Gently clean the thermocouple tip and pilot orifice. Ensure the pilot flame engulfs the thermocouple.
• Replace: If the pilot extinguishes after releasing the gas control knob, replace the thermocouple/thermopile with the correct length.
• Drafts: Correct nearby drafts that can blow out a marginal pilot flame.

If the pilot still drops out, the gas valve or safety controls may be faulty—seek professional service.

Cost And Time Estimates For Common Repairs

Costs vary by region, brand, and accessibility. The ranges below reflect typical U.S. residential service pricing for gas furnaces.

If a heat exchanger is compromised, many homeowners choose full furnace replacement for warranty and efficiency benefits, especially on older units.

How “Short Cycling” Wastes Energy And Money

Short cycling—when a furnace frequently starts and stops—reduces efficiency, increases wear, and can raise utility bills. Ignition and post-purge phases use energy without delivering steady heat, and components like igniters and motors wear out faster.

Fixing root causes such as airflow, venting, or control settings improves comfort and equipment life. For persistent cases, a professional load calculation (Manual J) can confirm if the furnace is oversized relative to the home.

Preventive Maintenance To Keep The Furnace Lit

• Filters: Check monthly during heating season. Replace 1-inch filters every 1–3 months; thicker media every 3–12 months per use and MERV rating.
• Annual service: Have a pro clean burners, flame sensor, and blower, verify combustion, inspect heat exchanger, and test safeties.
• Vents: Clear intake/exhaust terminations before storms. Maintain adequate clearance from shrubs and snow.
• Condensate: Flush the trap and line each fall. Add algaecide tablets if recommended by the manufacturer.
• Thermostat: Set appropriate cycles-per-hour (3–5 for forced air). Avoid locating thermostats near heat sources or drafts.
• Ducts: Open enough supply and return paths; consider professional static pressure testing to prevent high-limit trips.
• Surge protection: A whole-home surge protector can protect sensitive control boards.

What The Flame Should Look Like

A healthy natural gas flame is mostly blue with small, stable inner cones. Slight yellow tipping is acceptable, but large lazy yellow flames indicate incomplete combustion or dirty burners.

On LP, the flame is similarly blue but may be shorter. If flames lift off the burner or roar, the gas pressure or primary air may be incorrect—call a technician to adjust.

When The Furnace Lights But The Blower Runs Late

Blower-on delays are normal to prevent blowing cold air. However, if the blower comes on late, the heat exchanger can overheat and trip the limit, shutting burners.

Causes include incorrect blower delay settings, weak blower motor, or dirty heat exchanger. A professional can adjust fan profiles, verify amperage, and clean the exchanger.

Thermostat Settings That Reduce Short Cycling

• Cycles-per-hour: Set to 3–5 for gas furnaces. Higher numbers can cause frequent on/off cycles.
• Heat differential (swing): A slightly larger differential reduces rapid cycling while maintaining comfort.
• Adaptive recovery: Enable only if compatible; some models overshoot and cycle more on older furnaces.
• Location: Avoid direct sunlight, nearby supply registers, or exterior walls.

Why A Furnace Works Then Stops After 30–60 Seconds

This timing often points to flame-sensing, pressure switch, or ignition issues. If it runs 5–10 seconds only, suspect the flame sensor. If it barely lights or puffs out, examine venting or gas pressure. If it runs two to five minutes, airflow and high-limit trips become more likely.

Use the error code and observed timing to narrow the diagnosis and avoid unnecessary parts replacement.

Venting And Combustion Air Tips

• Clearances: Keep terminations the distances specified from windows, doors, and grade to prevent recirculating exhaust.
• Screening: Only use manufacturer-approved screens to prevent lint or ice buildup.
• Combustion air: Sealed-combustion furnaces pull air from outside; open-combustion units need adequate indoor air. Tight homes may require dedicated air inlets.
• Shared spaces: Avoid chemical vapors (bleach, paints) near the furnace; corrosive air can damage exchangers and sensors.

DIY Flame Sensor Cleaning: Detailed Steps

1. Turn off the furnace switch and shut off the gas valve.
2. Remove the burner access panel. Locate the flame sensor—a thin metal rod in front of a burner.
3. Remove the mounting screw and gently pull the sensor out.
4. Lightly polish the rod with a clean emery cloth or fine steel wool until shiny. Do not scrub the porcelain.
5. Wipe with a dry cloth, reinstall, and tighten the screw. Restore power and gas.
6. Run a heat cycle and observe. If the symptom persists, further diagnosis is required.

Airflow And Static Pressure: Why It Matters

High static pressure makes the furnace run hotter and trip limits. Causes include undersized ducts, restrictive filters, closed registers, and dirty coils. The result is a furnace that keeps shutting off and restarting.

A professional can measure external static pressure, compare to the furnace’s rated maximum, adjust blower speed, clean the coil, and recommend duct changes or less restrictive filtration to keep burners lit consistently.

Comparing Symptoms To Likely Causes

Smart Thermostats And Power Issues

Some smart thermostats steal power from furnace circuits, causing erratic behavior. Symptoms include relay chatter, short cycling, or control board resets that make the furnace flame go out.

Solution: Use a proper C-wire or a manufacturer-approved power extender. Verify the furnace transformer capacity and avoid mixing incompatible accessories on the 24V circuit.

What A Professional Technician Will Do

• Test sensors: Measure flame rectification microamps, pressure switch operation, and limit/rollout continuity.
• Combustion analysis: Measure O2/CO2, CO, and draft to verify safe, efficient burning.
• Electrical diagnostics: Check igniter resistance, inducer and blower amperage, capacitor microfarads, and board outputs.
• Mechanical inspection: Clean burners, verify heat exchanger integrity, and inspect venting and condensate systems.
• Corrections: Adjust gas pressure, re-seat grounds, update firmware (when applicable), and replace failing components.

Frequently Asked Questions

Why Does My Furnace Keep Shutting Off After A Few Minutes?

This is usually a high limit switch opening from poor airflow—dirty filter, closed registers, or a dirty coil. It can also be venting or pressure switch issues on high-efficiency furnaces.

Can I Run My Furnace Without A Filter To Test It?

Briefly for testing only, but do not operate long-term without a filter. Dust can foul the blower, coil, and flame sensor, leading to more shutdowns.

How Often Should I Clean The Flame Sensor?

Annually during maintenance is typical. If your furnace short cycles with a 5–10 second flame-out, cleaning sooner can help.

Is A Yellow Flame Dangerous?

Predominantly yellow, lazy flames indicate incomplete combustion, which can produce excess carbon monoxide. Shut down and schedule service.

What If The Furnace Keeps Locking Out?

Most controls allow several retries before lockout. Power cycling can reset temporarily, but repeated lockouts indicate a fault that needs diagnosis.

Key Takeaways To Keep Heat Reliable

• Start simple: Check thermostat, power, filter, and vent terminations first.
• Most common cause: A dirty flame sensor often makes a furnace won’t stay lit after 5–10 seconds.
• Airflow matters: High static pressure and clogged filters trigger high-limit shutdowns.
• Condensing models: Keep condensate lines flowing and PVC clear of ice and debris.
• Safety first: If there’s gas odor, CO alarm, rollout, or soot, shut down and call a pro.

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