Furnace Not Starting: Troubleshooting Guide

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Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

If your furnace won't start, first check the power supply and thermostat settings. Ensure the circuit breaker hasn't tripped and the gas valve is open. If the inducer motor runs but the furnace doesn't ignite, you may need to replace the flame sensor or igniter.

Confirm the symptom

Furnace doesn't turn on at all
No heat despite thermostat settings
No clicking or fan noise when calling for heat
Thermostat shows power but furnace is silent

Tools and prep

Screwdriver (flathead and Phillips)
Multimeter (for voltage checks)
Flashlight
Thermometer (for temperature checks)
Safety gloves and goggles
Owner's manual for furnace model

Step-by-step troubleshooting

Check Power Supply
Do: Ensure the furnace is plugged in and the circuit breaker hasn't tripped.
Observe: Power indicator on furnace or thermostat may be off.
Means: Power interruption is the issue.
Next: Reset the breaker if tripped.
Verify Thermostat Settings
Do: Set thermostat to 'Heat' and increase the temperature above room temperature.
Observe: Thermostat may not send a signal to the furnace.
Means: Incorrect thermostat settings or malfunction.
Next: Test with a backup thermostat if available.
Inspect Gas Valve
Do: Check the gas valve to ensure it's fully open.
Observe: Gas valve handle should be parallel to the pipe.
Means: Gas supply is restricted.
Next: Turn the valve to the open position.
Check Inducer Motor
Do: Listen for the inducer motor to start when the thermostat calls for heat.
Observe: Motor may run but not proceed to ignition.
Means: Possible issue with the flame sensor or igniter.
Next: Inspect flame sensor and igniter.
Test Flame Sensor
Do: Use a multimeter to test the flame sensor for continuity.
Observe: No continuity indicates a faulty sensor.
Means: Flame sensor needs replacement.
Next: Replace the flame sensor.
Inspect Igniter
Do: Check the igniter for cracks or damage using a flashlight.
Observe: Igniter may glow but not ignite the gas.
Means: Faulty igniter requires replacement.
Next: Replace the igniter.

Common causes

Cause	Likelihood	Difficulty	Part often needed
Tripped Circuit Breaker Power interruption prevents furnace operation.	Common	Easy	None
Incorrect Thermostat Settings Thermostat not signaling the furnace to start.	Common	Easy	None
Closed Gas Valve Gas supply is restricted, preventing ignition.	Sometimes	Easy	None
Faulty Flame Sensor Sensor fails to detect flame, causing safety shutdown.	Sometimes	Medium	Flame Sensor
Failed Igniter Igniter doesn't create a spark to ignite the gas.	Sometimes	Medium	Igniter
Dirty Air Filter Restricted airflow causes furnace to shut down for safety.	Sometimes	Easy	Air Filter
Failed Limit Switch Switch prevents operation if overheating or airflow issues occur.	Rare	Medium	Limit Switch
Failed Control Board Board fails to send signals to start the furnace.	Rare	Hard	Control Board

Parts matching

Common parts needed for a furnace not starting:

Flame Sensor
Igniter
Air Filter
Thermostat
Gas Valve
Limit Switch
Control Board

Purchase parts specific to your furnace model for compatibility.

When to call a professional

Furnace makes loud noises or strange sounds
Gas smell is detected
Furnace starts but doesn't produce heat
Error codes persist after troubleshooting
You're uncomfortable working with electrical components
Issues recur after repairs

FAQ

Why won't my furnace start even though the thermostat is set correctly?

The furnace may have a power issue, a faulty flame sensor, or a closed gas valve. Check these components first.

Can a dirty air filter prevent the furnace from starting?

Yes, a very dirty filter can restrict airflow, triggering a safety shutdown. Replace or clean the filter.

How do I know if the igniter is faulty?

If the igniter glows but doesn't ignite the gas, it's likely faulty and needs replacement.

Is it safe to reset the circuit breaker for the furnace?

Yes, resetting a tripped breaker is safe. If it trips again, consult a professional.

What should I do if I smell gas when trying to start the furnace?

Immediately shut off the gas valve, ventilate the area, and call a professional HVAC technician.