How to Light a Gas Furnace Safely and Troubleshoot Common Issues

Lighting a gas furnace can restore heat quickly, but it requires care, the right steps, and awareness of common problems. This guide explains pilot light and electronic ignition procedures, safety checks, troubleshooting tips, and when to contact a professional, tailored to U.S. households and building codes.

Task	Estimated Time	Safety Note
Pre-checks and smell test	5 minutes	Do Not Ignite If Gas Smell Present
Pilot Light Relighting	5–15 minutes	Follow furnace label instructions
Electronic Ignition Reset	2–10 minutes	Use manufacturer reset procedure
Basic Troubleshooting	15–45 minutes	Shut off gas if unsure

How Gas Furnaces Work

Gas furnaces heat air by burning natural gas or propane in a combustion chamber and transferring heat through a heat exchanger to a blower-driven duct system. Pilot lights or electronic igniters provide the ignition source, and safety devices like thermocouples, flame sensors, and limit switches ensure safe operation.

Safety First: Before Attempting To Light A Furnace

Always prioritize safety by performing basic checks before attempting to light a gas furnace. If a strong gas odor is present, evacuate the building and call the gas company or 911.

• Verify adequate ventilation and clear combustible materials away from the furnace.
• Shut off electrical power to the furnace to prevent accidental fan or ignition operation during inspection.
• Check the gas shutoff valve to ensure it is accessible and operable.
• Inspect for signs of leaks, rusted components, or rodent damage to wiring and gas lines.

Pilot Light Systems: Step-By-Step Relighting

Older furnaces commonly have a standing pilot light and manual valve. Follow the manufacturer label or the furnace’s user manual; the following general steps apply to most units.

1. Turn Off Gas And Power: Set the thermostat to off and switch the furnace power to off at the breaker or switch.
2. Wait: Allow five minutes after shutting off gas to let any residual gas dissipate.
3. Locate Controls: Find the gas control knob, usually labeled Off, Pilot, and On. The pilot assembly and instructions are typically on the furnace front panel.
4. Set To Pilot: Turn the control to Pilot and press/hold it down to open the pilot gas flow.
5. Ignite: While holding the control, use the furnace’s built-in igniter button or a long-stem lighter to ignite the pilot. Continue holding the control for 20–60 seconds so the thermocouple heats up.
6. Release And Turn On: Release the control knob; if the pilot stays lit, turn to On and restore power. If it won’t stay lit, repeat steps or inspect the thermocouple.

Common Pilot Light Problems

Pilot lights that will not stay lit typically point to a failing thermocouple, a dirty pilot orifice, or a misaligned flame. Thermocouple replacement is a common homeowner repair if comfortable with basic tools; otherwise contact a technician.

Electronic Ignition Systems: Reset And Restart

Most modern furnaces use electronic ignition (hot surface ignitor or intermittent pilot). These systems require different handling than standing pilots and often have reset or diagnostic procedures.

• Consult the furnace manual for the specific reset button or thermostat sequence.
• Reset often involves setting the thermostat to off, turning power to off, waiting 30 seconds, restoring power, and then setting the thermostat to a heating call.
• If the furnace attempts to ignite but fails repeatedly, check the ignitor for visible cracks or breakage and the flame sensor for dirt.

Hot Surface Ignitor And Flame Sensor Maintenance

Hot surface ignitors are fragile ceramic elements that glow to light burners; they are easily damaged by handling. Flame sensors detect a flame and shut off gas if no flame is present. Cleaning a flame sensor with a fine abrasive pad can often restore reliable operation.

Basic Troubleshooting Checklist

When a furnace won’t light or run, follow a systematic troubleshooting approach to identify common failures before calling a pro.

1. Thermostat: Verify batteries, correct mode, set temperature above room temp, and confirm wiring if recently replaced.
2. Power And Switches: Confirm furnace switch and circuit breaker are on.
3. Gas Supply: Ensure the gas valve near the furnace is open and other gas appliances are functioning.
4. Pilot/Ignitor: Try relighting the pilot or observe the electronic ignition sequence and note error codes on the control board.
5. Air Filter: Replace dirty filters; clogged filters can trigger limit switches and prevent operation.
6. Vents And Exhaust: Check for blocked flue, bird nests, or snow buildup around outdoor vents for direct-vent systems.

Interpreting Error Codes And LED Indicators

Many modern furnaces display LED flash codes on the control board for diagnostics. The furnace manual or a sticker on the furnace usually lists code meanings. Record the blink pattern and reference the manual to identify issues like pressure switch failure, ignition failure, or limit switch trips.

When To Call A Professional

Some situations require licensed HVAC technicians or gas utility intervention. Call a professional when there is a strong gas odor, repeated ignition failures, suspected cracked heat exchanger, carbon monoxide alarm activation, or inability to isolate the gas supply.

• Persistent pilot/ignition failure after cleaning and reseating components.
• Irregular burner flame color (yellow or flickering suggests incomplete combustion).
• Evidence of soot, rust, or heat exchanger cracks—these have safety implications.
• Complex electrical or gas valve replacements beyond basic homeowner competency.

Carbon Monoxide Risks And Detection

A malfunctioning furnace can produce carbon monoxide (CO), an odorless, colorless gas that can be deadly. Homeowners should install CO detectors near sleeping areas and the furnace, replace batteries annually, and follow detectors’ replacement timelines.

Maintenance Tips To Prevent Relighting Problems

Regular preventive maintenance reduces the frequency of lighting failures and improves efficiency. Schedule at least annual professional tune-ups for combustion analysis, heat exchanger inspection, and safety control testing.

• Replace or clean air filters every 1–3 months depending on usage.
• Keep the furnace area clear and maintain proper clearance for combustion air supply.
• Lubricate blower bearings when applicable and inspect belts and electrical connections.
• For direct-vent systems, keep outdoor vents clear of debris and ice.

DIY Tools And Parts Commonly Used

Homeowners often use a few basic tools and replacement parts for simple furnace maintenance and relighting tasks. Always disconnect power and follow manufacturer instructions.

• Long-stem lighter or built-in igniter for pilot relighting.
• Adjustable wrench, nut drivers, and screwdrivers for access panels and control adjustments.
• Replacement thermocouple, flame sensor, or hot surface ignitor sized to the furnace model.
• Fine abrasive pad for cleaning flame sensors and contact cleaner for electrical connectors.

Legal And Code Considerations In The United States

Furnace installation and major repairs are subject to local building codes and often require licensed technicians. Homeowners should consult local regulations for gas appliance work and obtain permits when required. Gas utilities may require customers to use licensed contractors for certain types of gas line or appliance work.

Cost Expectations For Repairs And Service

Costs vary widely by region and furnace type. Typical expenses include thermocouple replacement ($75–$200 parts and labor), hot surface ignitor replacement ($150–$350), and full diagnostic service ($80–$200). Major repairs like heat exchanger replacement can be several hundred to thousands of dollars and may justify replacement of an older unit.

Frequently Asked Questions

Why Won’t My Pilot Stay Lit?

Usually a failing thermocouple, a weak pilot flame, or a dirty pilot orifice. Thermocouple replacement often resolves the issue.

Can A Furnace Be Turned On During A Power Outage?

Most modern furnaces require electrical power for blowers and electronic ignition and will not operate during a power outage unless paired with a generator or battery backup designed for HVAC loads.

Is It Safe To Light A Furnace If The Gas Smells Slightly?

No. Any noticeable gas odor warrants evacuation and immediate contact with the gas utility or emergency services. Do not attempt to ignite appliances.

Key Takeaways For Safe Furnace Lighting

• Always perform a gas smell check and ensure power is off before working on a furnace.
• Follow the manufacturer instructions for pilot relighting or ignition reset sequences.
• Regular maintenance and clean flame sensors increase reliability and safety.
• Call a licensed HVAC technician for persistent issues, suspected CO, or heat exchanger concerns.

For detailed instructions specific to a furnace model, consult the owner’s manual or the manufacturer’s website. When in doubt about safety or the source of a problem, contact a licensed technician or the gas utility for assistance.