Meta Description: If a high-efficiency furnace’s main burner lights and then shuts off, this guide explains likely causes, step-by-step diagnosis, safe DIY checks, and when to call a pro—covering flame sensors, pressure switches, condensate drains, venting, gas supply, and limit trips.
When a high-efficiency furnace burner lights, runs briefly, and shuts off, the issue is usually safety-related. This guide explains why it happens, how to pinpoint the timing pattern, and what fixes work. It covers flame proving, airflow, venting, condensate drainage, and gas supply so the heat stays on safely.
What It Means When Burners Light Then Go Out
High-efficiency furnaces follow a precise sequence: call for heat, inducer pre-purge, pressure switch closure, ignition, gas valve opens, burners light, flame proven, then blower starts. If the main burner shuts off after initial ignition, the control board is not satisfied with a safety or proof signal.
Most shutdowns fall into two buckets: flame proving failures within a few seconds, or limit and airflow issues after one to several minutes. A third pattern is short cycling, where burners start and stop repeatedly during a single thermostat call.
Modern boards monitor flame with a flame sensor and look for correct pressure, temperature, and safe venting. When a parameter falls out of range, the board shuts the gas valve and often retries before entering soft or hard lockout.
Diagnose By Timing: Seconds Versus Minutes
Burner Shuts Off After 2–10 Seconds
When burners shut down in seconds, the board likely did not “prove” flame. A dirty or mispositioned flame sensor, poor ground or reversed polarity, weak gas carryover between burners, or a dropping pressure switch signal can cause this.
- Flame Sensor: Oxide buildup prevents microamp flow. Cleaning is often effective.
- Grounding/Polarity: Loose ground or reversed hot/neutral affects flame rectification.
- Low Gas Flow: Weak flame fails to reach the sensor or carry across burners.
- Pressure Switch: Inducer vacuum marginally closes switch, then it reopens after ignition.
- Blocked Intake/Exhaust: Vent restrictions change pressure once flame starts.
Burner Shuts Off After 1–5 Minutes
Burners that run for a minute or more and then stop typically trip the high limit switch, signaling overheating from airflow problems. The board cuts gas to protect the heat exchanger.
- Dirty Filter or closed registers increase static pressure and temperature.
- Blower Issues: Fan never starts, runs too slow, or wrong speed selected.
- Return/Coil Restrictions: Dirty evap coil or blocked returns reduce airflow.
- Secondary Heat Exchanger restriction in condensing furnaces raises temperature.
Rapid Short Cycling Every Few Minutes
Repeated on-off cycles without reaching setpoint point to oversizing, a thermostat problem, venting affected by wind/ice, or intermittent safety signals. This wastes energy and stresses parts.
- Thermostat: Loose wires, weak batteries, or incorrect heat cycle settings.
- Venting: Frosted PVC termination or wind-induced pressure swings.
- Condensate: Trap fills and intermittently blocks the drain.
Quick Troubleshooting Checklist
- Observe Timing: Does the flame drop in seconds or after minutes? Note any blower behavior.
- Watch the Board LED: Count flashes and check the sticker or manual for code meanings.
- Check the Filter: Replace if dirty. Ensure supply and return vents are open.
- Inspect Vent Terminations: Remove snow, leaves, nests, or ice from intake/exhaust.
- Confirm Condensate Drain: Clear full traps, kinks, or frozen lines; verify consistent flow.
- Clean Flame Sensor: If safe and accessible, polish gently and reinstall firmly.
- Check Door Switches and Panels: Ensure panels are secure to engage safety interlocks.
- Thermostat Basics: Fresh batteries, correct heat mode, and a higher setpoint test.
Common Causes And Practical Fixes
Dirty Or Faulty Flame Sensor
The flame sensor detects flame via a microamp DC signal produced by flame rectification. Oxide on the rod reduces signal, so the board shuts gas within seconds.
- Signs: Burner lights for a few seconds, then shuts off, often retrying several times.
- DIY: Power off. Remove the sensor. Lightly polish the rod with fine abrasive pad or steel wool. Wipe clean. Reinstall firmly.
- Pro: If microamps remain low, replace sensor or assess board/ground problems.
Grounding, Polarity, And Wiring Issues
Flame proving requires a solid ground path and correct line polarity. Loose ground lugs, corroded connections, or reversed hot/neutral reduce flame signal.
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- DIY: With power off, tighten ground screws on furnace chassis and verify secure sensor wiring. Ensure outlet polarity if the furnace uses a corded connection.
- Pro: Measure flame sensor microamps and correct wiring at the service disconnect.
Venting Blockages Or Termination Problems
High-efficiency furnaces use PVC intake and exhaust. Snow, ice, debris, or insect nests can restrict airflow, changing pressure once burners fire and causing shutdown.
- Signs: Inducer runs, flame starts, then pressure fluctuations shut gas.
- DIY: Clear visible obstructions at terminations. Ensure proper spacing and orientation of pipes per label.
- Pro: Inspect for sagging pipes, excessive length/elbows, or water pooling; verify combustion analysis.
Condensate Drain Or Trap Issues
Condensing furnaces create water that must drain. A clogged trap, hose, or condensate pump can back up and disrupt pressure, extinguishing the flame or opening the pressure switch.
- Signs: Gurgling, slow drip, water near furnace, intermittent firing on humid or long cycles.
- DIY: Power off. Check trap and hoses for sludge. Flush with warm water and a small amount of vinegar. Ensure downhill slope and unfrozen lines.
- Pro: Replace damaged trap assemblies or pumps; verify correct venting pitch to avoid pooling.
Weak Or Sticking Pressure Switch
The pressure switch proves inducer draft. Marginal switches may close at start but open when flame changes pressure, shutting gas.
- Signs: Error code for pressure switch open or stuck. Burners go out within seconds.
- DIY: Inspect rubber tubing for cracks, kinks, or water. Clear the inducer port with a small drill bit or wire (do not enlarge).
- Pro: Verify inducer vacuum, switch setpoint, and vent resistance; replace the switch if it fails under spec.
Airflow Restrictions And High Limit Trips
Poor airflow raises heat exchanger temperature and triggers the high limit. The board stops the burners to prevent overheating.
- Signs: Burners run one to five minutes, then shut off; blower often continues to cool.
- DIY: Replace filters. Open supply/return registers. Check for crushed ducts or blocked returns. Clean visible dust on grilles.
- Pro: Measure static pressure, adjust blower speed, clean coils, and verify temperature rise matches the nameplate.
Blower Motor Or Control Issues
If the blower does not start on time or runs too slowly, heat builds up and trips the limit. ECM blowers may misbehave due to control or programming faults.
- Signs: No airflow feel at vents before burner shutdown. Delayed or erratic blower start.
- DIY: Listen for blower start. Ensure fan door switch is engaged. Do not bypass safeties.
- Pro: Diagnose capacitor (PSC motors), ECM module, relay, or control board fan timer.
Gas Supply, Regulator, And Manifold Pressure
Low incoming gas pressure, a failing gas valve, or improper manifold pressure can produce weak flame that fails to carry to the sensor or maintain combustion.
- Signs: Lazy, lifting, or noisy flame; burners light but drop quickly; other gas appliances affected.
- DIY: Confirm that other gas appliances work normally; ensure the gas valve handle is fully open.
- Pro: Measure inlet and manifold pressure; adjust regulator per nameplate (typical natural gas 3.5 in. w.c., LP 10–11 in. w.c.).
Ignition Source And Carryover
Hot surface igniters and spark systems must reliably light all burners. Delayed ignition or poor cross-lighting can cause the sensor to “see” weak flame, shutting off gas.
- Signs: Repeated clicks, delayed woof, or flame only on one burner.
- DIY: Visual check for rust/debris on burner crossovers; gently vacuum without disturbing alignment.
- Pro: Clean or replace burners, verify igniter resistance, and set correct gap/position for spark.
Thermostat And Control Wiring
Loose thermostat connections or low batteries can cause premature shutdowns or short cycling. Miswiring between R, W, G, and C can confuse heat calls.
- DIY: Replace batteries, tighten low-voltage connections, and ensure correct heat mode.
- Pro: Test with a jumper at the furnace (R to W) to isolate thermostat issues safely.
Safety Switches: Rollout And Limits
Flame rollout switches trip if flame escapes the burner area. This indicates serious combustion or heat exchanger issues and will shut the burner down immediately or prevent restart.
- Signs: Tripped manual-reset switch, scorch marks, odor, or sooting.
- Action: Do not reset without finding the cause. Call a professional immediately.
Diagnostic Table: Symptoms, Likely Causes, And Quick Checks
| Symptom Timing | Likely Cause | Quick Homeowner Checks | Pro-Level Tests |
|---|---|---|---|
| Shuts Off In 2–10 Seconds | Dirty flame sensor; poor ground; low gas; pressure switch opens | Clean sensor; secure panels; clear vents; observe flame quality | Measure flame sensor microamps; manifold pressure; verify polarity |
| Runs 1–5 Minutes, Then Off | High limit trip from airflow restriction | Replace filter; open vents; look for blocked returns | Static pressure and temperature rise; blower speed settings |
| Cycles Every Few Minutes | Thermostat or venting issue; condensate intermittent | Change thermostat batteries; clear vent frost; flush trap | Combustion analysis; vent sizing; condensate pump performance |
| Won’t Relight After Tries | Lockout after failed flame proving | Power-cycle furnace once; observe LED code | Control board diagnostics; wiring continuity |
Key Specifications And Normal Ranges
Knowing normal specs helps separate ordinary operation from a fault. Always check the unit’s data plate and manual for exact values.
| Parameter | Typical Range | Notes |
|---|---|---|
| Flame Sensor Signal | 2–6 µA DC | Some brands accept >1 µA; stable signal prevents dropouts |
| Manifold Pressure (NG) | ~3.5 in. w.c. | Verify against nameplate for your model |
| Manifold Pressure (LP) | 10–11 in. w.c. | Regulator and orifice differ from NG |
| Pressure Switch Setpoint | ~0.40–1.00 in. w.c. vac | Varies; printed on switch label |
| Blower On-Delay | 30–90 seconds | Board-controlled; prevents cold blow |
| Flame Trial Period | 4–10 seconds | Board shuts gas if flame is not proven |
| Temperature Rise | 30–60 °F | Must match nameplate to avoid limit trips |
Safe DIY Steps To Try First
Many “burners light then shut off” issues have simple causes. The steps below are generally safe for homeowners. If uncomfortable at any point, stop and call a professional.
- Replace The Air Filter: Use the correct size and orientation. A clogged filter commonly trips the limit.
- Clean The Flame Sensor: Power off. Gently polish the rod, reattach firmly, and ensure the wire is snug.
- Clear Vent Terminations: Remove obstructions at intake and exhaust. Maintain clearance from shrubs and snow.
- Flush The Condensate Trap: Disconnect and rinse with warm water. Reassemble with proper seals and slopes.
- Check Thermostat: Fresh batteries, correct mode, and a higher setpoint to force a call for heat.
- Secure Access Panels: Ensure the blower door is fully closed to engage the door switch.
What A Technician May Do
When the problem persists, a licensed technician performs deeper diagnostics that require tools and training. This protects equipment and ensures safe operation.
- Measure Flame Sensor Microamps: Adjust grounding, move sensor position, or replace the sensor if weak.
- Combustion Analysis: Confirm safe CO and O2 levels and proper excess air.
- Verify Gas Pressures: Check inlet and manifold pressure and adjust regulators.
- Evaluate Vent System: Inspect pitch, length, elbows, and joint seals to manufacturer specs.
- Check Static Pressure And Blower Settings: Balance speeds and ducts; clean coils; correct high static.
- Test Pressure Switch And Inducer: Confirm reliable operation across ignition and steady state.
Costs And Repair Expectations
Repair pricing varies by region and brand, but typical ranges help set expectations. Addressing problems early reduces wear and avoids repeated lockouts.
- Service Call And Diagnosis: $90–$180, often applied to repairs.
- Flame Sensor Cleaning/Replacement: $0–$20 for DIY cleaning; $120–$300 for replacement including labor.
- Pressure Switch Or Condensate Trap: $150–$400 depending on parts and access.
- Igniter Replacement: $180–$350 for hot surface igniters.
- Blower Or Control Board Work: $250–$900+ depending on ECM modules and complexity.
Safety Notes You Should Not Ignore
Furnace safeties are designed to shut off gas to protect occupants and equipment. Bypassing them is dangerous. Address root causes instead.
- Smell Gas Or See Soot? Shut off the system and call a professional immediately.
- Do Not Jump Out Safety Switches: This risks fire, carbon monoxide, or equipment damage.
- Carbon Monoxide Alarms: Install and test CO alarms on every level of the home.
How Weather And Installation Affect Performance
Cold snaps can expose marginal systems. Heavy snowfall can block PVC pipes. Wind can momentarily upset pressure balance at terminations, especially if poorly located.
- Clearances: Maintain manufacturer-specified distances from grade, roofs, and openings.
- Pipe Pitch: Exhaust must pitch back to the furnace; intake generally pitches away to prevent water entry.
- Length/Elbow Limits: Exceeding equivalent length reduces draft and can cause pressure switch issues.
Reading Your Furnace’s LED Codes
Most furnaces have an LED behind a sight glass. When burners shut off, note the number and pattern of flashes. Use the unit’s door label to decode and guide next steps.
- Pressure Switch Open: Check venting, inducer, hoses, and the switch.
- Limit Or Rollout Open: Investigate airflow or combustion issues; rollout demands immediate professional attention.
- Flame Not Proven: Focus on flame sensor, ground, polarity, gas carryover, or ignition source.
Preventive Maintenance To Avoid Short Cycling
Routine maintenance keeps the flame on and fuel bills down. High-efficiency systems also need condensate and vent attention not found on older units.
- Filters: Replace every one to three months, or per manufacturer guidance.
- Annual Tune-Up: Clean burners and sensor, check pressures, analyze combustion, and test safeties.
- Condensate Care: Flush traps each season; check pump operation before heating season.
- Vent Inspection: Verify clear terminations and stable supports; remove pest screens that clog.
- Electrical Integrity: Tighten grounds and neutrals; consider surge protection.
FAQ: Burners Light Then Go Out
Why Does My Furnace Flame Go Out After A Few Seconds?
Usually the flame sensor is not proving flame. Dirt on the sensor, poor grounding, reversed polarity, or inadequate flame carryover can cause the board to close the gas valve for safety.
Why Does My Furnace Shut Off After A Few Minutes?
That pattern points to a high limit trip caused by restricted airflow. A dirty filter, blocked returns, closed supply registers, or blower issues commonly trigger it.
Can A Clogged Condensate Trap Shut My Furnace Down?
Yes. A blocked trap or hose changes the pressure in the combustion chamber, opening the pressure switch during or after ignition and shutting off the gas.
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Do I Need To Replace The Flame Sensor Or Just Clean It?
Often a gentle cleaning restores proper microamps. If the signal remains low after cleaning and confirming ground, replacement is inexpensive and effective.
Is It Safe To Reset A Rollout Switch?
No—do not reset without professional evaluation. Rollout indicates dangerous combustion or heat exchanger issues that require immediate service.
Step-By-Step Diagnostic Flow
- Confirm Symptoms: Note whether shutdown occurs in seconds or minutes. Observe LED codes.
- Restore Airflow: Replace the filter and open all vents. Check returns for obstruction.
- Vent And Drain: Clear intake/exhaust terminations and flush the condensate trap and lines.
- Flame Sensor: Clean carefully and resecure. Verify the wire is intact and connected.
- Power Cycle: Turn off power for 60 seconds, then restart and watch the sequence.
- Evaluate: If the problem persists, call a professional to test flame microamps, pressures, and controls.
Why This Problem Is Common On High-Efficiency Units
Condensing furnaces rely on tight venting and drainage. Small changes in pressure or water buildup can upset the balance more readily than in older, non-condensing models.
- More Sensors, More Protection: Additional safeties reduce risk but can create nuisance trips when maintenance is overdue.
- Venting Sensitivity: Long PVC runs and outdoor conditions affect draft stability.
- Condensate Management: Traps and pumps require periodic cleaning to prevent shutdowns.
Signals You Should Call A Professional Now
- Flame Rollout Or Soot: Immediate hazard; shut down system.
- Repeated Lockouts: The furnace stops trying and needs diagnosis.
- Gas Odors: Evacuate and call the gas utility or a licensed contractor.
- ECM Or Control Faults: Advanced electronics require proper tools.
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Key Takeaways
- Seconds Shutoff = Flame Proving: Clean the flame sensor, verify ground and polarity, and check vent and pressure switch behavior.
- Minutes Shutoff = Overheating: Fix airflow—filters, vents, blower function, and coil cleanliness.
- Don’t Ignore Condensate: Clear traps and pumps to stabilize pressure and combustion.
- Use LED Codes: They point directly to pressure, limit, or flame-proving faults.
- Safety First: Never bypass safeties; call a pro when the root cause is unclear.
Glossary
- Flame Sensor: A rod that confirms flame via microamp DC current through flame rectification.
- Pressure Switch: A vacuum-activated switch proving inducer-draft operation.
- High Limit Switch: Temperature safety that shuts burners if the furnace overheats.
- Inducer Motor: Draft fan that moves combustion gases through the heat exchanger and vent.
- Condensate Trap: Water seal that drains acidic condensate from the secondary heat exchanger.
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