What could cause a furnace to fail to stay lit after ignition?

A faulty flame sensor can lead to a furnace failing to stay lit after ignition because the flame sensor is responsible for detecting whether the flame is present once the gas has ignited. If the flame sensor is malfunctioning or dirty, it may not accurately sense the flame, causing the safety mechanism in the furnace to shut off the gas supply to prevent potential hazards such as gas buildup. This is a critical safety feature to avoid dangerous situations, and when the sensor does not function correctly, it results in the flame being extinguished shortly after ignition.

While other issues, like low gas pressure, incorrect thermostat settings, or a dirty burner assembly, can also impact the furnace's operation, they would typically manifest in different ways or prevent ignition entirely rather than causing the flame to go out after starting. For instance, low gas pressure might prevent proper ignition or lead to inconsistent heating, but the primary function of the flame sensor specifically addresses the safety and stability of the flame after it has ignited.