Furnace Burner Won’t Stay Lit in Greater Vancouver

A furnace that lights successfully but shuts off the burner within 3–10 seconds is experiencing a flame sensing failure — the control board is not receiving confirmation that a flame is present and shuts off the gas valve for safety.

What This Symptom Means

When the burner lights, the flame sensor (a metal rod in the burner flame) should generate a small microamp current confirming combustion is occurring. If this signal is absent or too weak, the board interprets it as an unsafe condition and closes the gas valve — extinguishing the flame.

Common Causes

• Dirty flame sensor — oxidation coating the sensor rod prevents current generation
• Failed flame sensor — rod has failed electrically
• Incorrect flame sensor position — not positioned in the burner flame path
• Grounding problem at the sensor mount affecting current reading
• Control board not reading the sensor signal correctly
• Flame sensor wiring damaged or disconnected

What Homeowners Can Check Safely

• Look at furnace status light — after burner shuts off, count blink pattern and compare to door label
• Note how many seconds the burner stays on — 3–5 seconds typically means flame sensor; longer may suggest a different fault

When to Call a Licensed Technician

Some furnace issues require a licensed HVAC technician. Do not attempt to open gas lines, replace internal components, or reset persistent lockouts without professional diagnosis.

• Flame sensor cleaning with fine steel wool or emery cloth — a standard maintenance step
• Flame sensor electrical testing with a multimeter in microamp mode
• Sensor replacement if cleaning does not resolve
• Control board inspection if sensor tests good
• Sensor wire and ground inspection

Frequently Asked Questions

How much does it cost to clean or replace a flame sensor?

Flame sensor cleaning is typically included in an annual tune-up. A separate service call for flame sensor cleaning usually takes under 30 minutes. Replacement sensors are inexpensive — the labour for a service call is the main cost. This is one of the more affordable furnace repairs.

Can I clean the flame sensor myself?

Some homeowners do clean the flame sensor themselves with light steel wool, but it requires safely accessing the furnace interior, identifying the correct rod (not the igniter), and reinstalling without bending or displacing it. If you are comfortable with basic maintenance and know your specific furnace model, it is a reasonable DIY task. When uncertain, a technician is the safer choice.

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