Furnace Won’t Stay Lit but Flame Sensor Is Not the Issue

The furnace not staying lit is a common problem that causes cold homes and higher energy bills. When the flame sensor is ruled out, troubleshooting must shift to other components and conditions that interrupt burner operation. This article explains likely causes, step-by-step diagnostics, and actionable repairs or professional steps to restore reliable furnace operation.

Symptom	Possible Causes	Typical Fix
Ignition lights then shuts off	Pilot/igniter, blocked vent, gas pressure, limit switch	Replace igniter, clear vent, adjust gas, replace limit switch
Burner cycles frequently	Thermostat, airflow restriction, dirty filter	Calibrate thermostat, replace filter, clear ducts
Short runtime then lockout	Flame rollout, pressure switch, inducer motor issue	Inspect for blockages, test switches, replace inducer

How Furnaces Normally Stay Lit

Modern gas furnaces use an ignition sequence controlled by the control board and safety devices. The thermostat calls for heat, the inducer motor runs to vent combustion gases, pressure and rollout switches confirm safety, the gas valve opens, and the igniter or pilot lights the burners. The flame sensor verifies flame presence to keep the gas valve energized.

Why Flame Sensor Gets Blamed Frequently

The flame sensor is a common culprit because it directly detects flame and is easy to clean. When a furnace lights briefly then shuts off, the flame sensor often shows corrosion or dirt, so many technicians or homeowners replace or clean it first. However, many other faults mimic the same short-run behavior.

Top Causes When The Flame Sensor Is Not At Fault

1. Faulty Igniter Or Pilot Assembly

Igniters in electronic ignition furnaces can crack or weaken and still light initially but fail to sustain a stable flame. A pilot-equipped furnace may have a weak pilot or thermocouple problem that behaves similarly.

Diagnostics

• Observe the ignition: does the hot surface igniter glow consistently before gas flow?
• Measure igniter resistance (compare to manufacturer spec) if safe and trained to do so.
• In pilot systems, check pilot flame size and thermocouple voltage.

Typical Fix

Replace a cracked igniter or adjust/replace a weak pilot assembly or thermocouple. This often restores stable ignition.

2. Gas Supply And Pressure Issues

Insufficient gas pressure or partial blockage in the gas line can allow lighting but not sustain full burner demand. A regulator malfunction or closed/partially closed valve can create intermittent flame behavior.

Diagnostics

• Check other gas appliances for reliable operation to rule out house-wide supply problems.
• Technician pressure-test the gas line and regulator at the furnace.

Typical Fix

Adjust or replace the regulator, clear line obstructions, or coordinate with the gas utility for low mains pressure.

3. Venting And Combustion Air Problems

Blocked or restricted exhaust vents, chimney obstructions, or inadequate combustion air can cause the furnace to shut down quickly for safety. Pressure switches detect proper venting and will open if conditions are unsafe.

Diagnostics

• Inspect exterior vent terminations for nests, debris, ice, or snow.
• Listen for unusual inducer motor sounds or repeated inducer cycles.
• Check pressure switch tubing for disconnections or blockage.

Typical Fix

Clear vent obstructions, repair vent piping, replace a failing inducer motor, or replace kinked/disconnected tubing to the pressure switch.

4. Faulty Pressure Switch Or Inducer Motor

The pressure switch proves the inducer motor has created the correct draft for safe combustion. A failing inducer motor may run but not generate enough vacuum, or the pressure switch contacts can become stuck or fail.

Diagnostics

• Measure vacuum or test switch continuity during startup to verify operation.
• Observe for delayed or noisy inducer motor start, or repeated cycling into lockout.

Typical Fix

Replace the pressure switch or inducer motor as needed. Ensure proper tubing and connections to the switch are clean and intact.

5. Limit Switch Or Overheat Protection Tripping

High-limit switches protect against overheating. Restricted airflow from dirty filters, closed vents, or blower failures can cause rapid shutdown after ignition due to high temperatures or inadequate heat exchange.

Diagnostics

• Check filter condition, blower operation, and ductwork for closed registers.
• Inspect the high-limit switch for continuity and correct resetting behavior.

Typical Fix

Replace the filter, repair or replace the blower motor, and replace a defective limit switch. Verify proper airflow and cleared return vents.

6. Thermostat Or Control Board Issues

Incorrect signals from the thermostat or a failing control board can interrupt the call for heat mid-cycle. Short circuits, loose wiring, or a board that times out due to detected faults will shut off gas flow.

Diagnostics

• Swap the thermostat with a known working unit or test by shorting R and W to simulate a call for heat.
• Inspect for error codes or LED blink patterns on the control board per the furnace manual.

Typical Fix

Repair thermostat wiring, reprogram or replace the thermostat, or replace the control board if error diagnostics point to board failure.

Step-By-Step Troubleshooting Guide

This section outlines a safe, prioritized approach to diagnose a furnace that lights then shuts off when the flame sensor is not the issue. Only qualified individuals should work on gas appliances; when in doubt, call a licensed HVAC technician.

1. Confirm Safety: Turn power and gas off before inspecting components and follow manufacturer safety guidelines.
2. Check Error Codes: Look at the control board LED or display for diagnostic codes and document them.
3. Inspect Vents: Check exterior vents for obstructions and ensure inducer motor runs at startup.
4. Observe Ignition Sequence: Note if the igniter glows consistently and if burners establish a stable flame.
5. Test Pressure Switch: Ensure the switch closes when the inducer runs; inspect tubing for blockages or disconnections.
6. Check Gas Supply: Verify other appliances and consult the gas utility if house pressure seems low.
7. Evaluate Airflow: Replace filters, inspect the blower, and check return air paths for restrictions.
8. Review Limit Switches: Test high-limit and rollout switches for correct operation.
9. Check Control Signals: Test thermostat operation and inspect control board connections and error codes.
10. Call A Pro: If tests point to inducer, gas valve, or control board failures, engage a licensed HVAC technician for repairs.

When To Replace Parts Versus Call A Technician

Minor items such as air filters and easily accessible igniters can be homeowner-handled with care. Components involving gas pressure, control boards, inducer motors, or complex wiring require a licensed technician. For safety and code compliance, gas valve and regulator replacements should not be attempted by unqualified persons.

Costs And Timeframes For Repairs

Costs vary by region and part. Typical ranges include: igniter replacement $100–$300, pressure switch $80–$200, inducer motor $300–$700, control board $200–$600, gas valve/regulator $200–$800. Labor and emergency service fees can increase totals. Most repairs are completed the same day by a service technician, while complex diagnostic issues may require multiple visits.

Maintenance Tips To Prevent Future Short Runs

• Replace filters every 1–3 months depending on usage and filter MERV rating to maintain airflow.
• Schedule annual professional tune-ups for inspection of ignition components, gas pressure, and safety switches.
• Keep vents clear of debris, bird nests, and snow to ensure proper venting.
• Monitor thermostat behavior and replace batteries or recalibrate if cycling anomalies appear.
• Maintain proper combustion air by avoiding blocking return grills and ensuring adequate make-up air in tightly sealed homes.

Safety Red Flags Requiring Immediate Action

• Strong gas odor: Evacuate and contact the gas utility or 911.
• Repeated burner lockouts with visible flame rollout or scorched components: Shut down and call a technician.
• Carbon monoxide alarm activation: Evacuate, ventilate, and call emergency services.
• Sparking, burning smells, or visible smoke from furnace components: Disconnect power and call a professional.

Resources And When To Call A Technician

Useful resources include the furnace manufacturer’s service manual, the utility company’s guidelines for gas safety, and the U.S. Consumer Product Safety Commission for appliance safety tips. Homeowners should call a licensed HVAC technician when diagnostics involve gas pressure, control boards, inducer motors, or any repair beyond basic filter or igniter replacement.

With systematic diagnostics and timely maintenance, most causes of a furnace that won’t stay lit (when the flame sensor is not the issue) can be identified and corrected, restoring reliable heating and safety.