Furnace starts but shuts off after a few seconds, repeatedly cycling without heating the space

Safety first: Turn off power and fuel before cleaning or adjusting burner components.

Steps:

• Clean flame sensor: Remove and clean the Danfoss flame rod or photocell as described in earlier steps, ensuring a clean sensing surface.
• Check air filter and vents: Replace a dirty filter and open all supply/return vents to improve airflow and prevent limit trips.
• Inspect flue and intake: Verify that vent and intake terminations are clear of obstructions and that flue joints are intact.
• Observe operation: After resetting, watch the burner through the inspection port. If the flame is unstable or noisy, stop and call a technician for combustion tuning.