Furnace Not Igniting? 4 Common Fixes

A furnace that will not ignite leaves you without heat, which is a real concern even in Phoenix where winter nights can drop into the 30s. This guide covers the four most common fixes, key safety rules, and the tools and time each repair takes.

A furnace that will not ignite leaves you without heat. This is a serious concern even in Phoenix, where winter nights can drop into the 30s. This guide walks you through the four most common ignition fixes, safety rules, and when to call a professional. If you also have cooling issues, check our guide on why your AC blows warm air for a parallel troubleshooting checklist.

Fix 1: Check the Thermostat

The thermostat starts the heating cycle. If it is not sending the right signal, the furnace will not ignite.

What to Check

• Make sure the thermostat is set to "Heat" and the temperature is at least 3°F above the current room temperature.
• Replace the batteries if the display is blank or dim.
• If it is a programmable thermostat, verify the schedule is not set to "Away" or "Sleep" mode.
• Check that the thermostat is level and not near drafts or heat sources that could cause false readings.

Estimated Time

5 to 15 minutes

Tools Needed

• Replacement batteries (AA or AAA)
• Screwdriver (if removing the thermostat cover)

Fix 2: Check the Pilot Light or Igniter

Older furnaces use a standing pilot light. Newer models use an electronic hot surface igniter or spark igniter.

Pilot Light (Older Furnaces)

• Check if the pilot flame is lit. If not, follow the manufacturer's relighting steps on the furnace panel.
• If the pilot will not stay lit, the thermocouple may be faulty. This safety device shuts off gas when the pilot goes out.
• Thermocouple replacement costs $150 to $300 with labor.

Electronic Igniter (Newer Furnaces)

• Listen for a clicking sound (spark igniter) or look for an orange glow (hot surface igniter) when the furnace tries to start.
• If there is no glow or click, the igniter is likely cracked or burned out.
• Igniter replacement costs $150 to $300 with labor.

Estimated Time

30 to 60 minutes (professional); DIY not recommended for gas parts

Tools Needed (Professional)

• Multimeter
• Nut driver set
• Replacement igniter or thermocouple

Fix 3: Check the Gas Valve

The gas valve controls gas flow to the burners. If it is closed or faulty, the furnace cannot ignite.

What to Check

• Find the gas valve on the gas supply line near the furnace. The handle should be parallel to the pipe (open). If it is perpendicular, it is closed — turn it to open.
• Check that other gas appliances in the home are working (stove, water heater). If none work, the issue may be with your gas supply — call your utility company.
• If the valve is open but no gas flows, the gas valve itself may be faulty and need replacement.

Estimated Time

15 to 30 minutes to diagnose; 1 to 2 hours for gas valve replacement (professional)

Tools Needed

• Adjustable wrench
• Gas leak detector solution
• Replacement gas valve (if needed)

Note: Gas valve replacement must follow NFPA 54 (National Fuel Gas Code). Only a licensed professional should do this repair.

Fix 4: Clean the Flame Sensor

The flame sensor is a safety device that confirms the burner has ignited. If it is dirty with carbon buildup, it cannot detect the flame and shuts off the gas valve — preventing ignition.

What to Do

• Turn off power and gas to the furnace.
• Find the flame sensor — a thin metal rod near the burner assembly, held by one screw.
• Remove it gently and clean it with fine-grit sandpaper (220-grit) or steel wool until the metal is shiny.
• Reinstall, restore power and gas, and test.

Estimated Time

15 to 30 minutes

Tools Needed

• 1/4" nut driver
• Fine-grit sandpaper (220-grit) or steel wool
• Clean cloth

Repair Cost and Time Summary

Fix	Estimated Cost	DIY?	Time
Thermostat check/repair	$0–$250	Yes	5–15 min
Pilot light / igniter	$150–$300	No (gas)	30–60 min
Gas valve	$200–$600	No	1–2 hours
Flame sensor cleaning	$80–$250	Yes (cleaning)	15–30 min

If you are weighing whether to repair or replace your furnace, our heat pump vs furnace comparison breaks down 10-year costs by climate zone. And if a failing AC compressor is part of your HVAC concerns, see our guide on compressor replacement costs for budgeting help.