Furnace lights but flame goes out after a few seconds; flame failure lockout.

Safety first: Turn off power and gas before servicing the burner area.

Steps:

• Locate the CTC flame sensor, a small metal rod mounted at the burner assembly with a single wire attached.
• Remove the mounting screw and gently pull out the sensor.
• Clean the metal rod with fine steel wool or a Scotch-Brite pad until shiny. Do not sand the porcelain insulator.
• Inspect the sensor wire and connector for corrosion or loose fit at the control board; clean and reseat as needed.
• Reinstall the sensor, ensuring it is properly positioned in the burner flame path.
• Restore power and gas, then call for heat and observe operation. If the flame still drops out, the sensor may be weak and should be replaced with a genuine CTC flame sensor.

Note: Persistent flame failures after sensor replacement may indicate gas valve or control board issues; contact a technician.