FLAME SENSOR

• The burners ignite, run for 3–8 seconds, then the flame goes out.
• The furnace may attempt this cycle 2–3 times before locking out.
• This is the #1 most common flame sensor symptom.

Tip: Before replacing the flame sensor, try cleaning it with fine sandpaper or steel wool. A dirty sensor is the most common furnace repair — and it's free to fix.

• The furnace starts and stops repeatedly over a short period.
• Each cycle may get shorter as the control board counts failed attempts.
• After 3 failed attempts, most furnaces will lock out for 1–3 hours.

• The LED on your furnace control board is blinking a specific pattern.
• Common codes: "flame sensed without call for heat" or "no flame detected."
• Check the label inside your furnace door for the code chart.

• After multiple lockout cycles, the furnace may stop attempting to light.
• You'll need to reset the furnace (turn power off/on) or wait for the lockout timer.
• If cleaning doesn't fix it, the sensor rod itself may be cracked or worn out.