If your furnace initiates the start cycle (powers on) but does not fire up, as it is supposed to, the problem is likely a faulty, damaged, or dirty ignitor/sensor. Unfortunately, this is a very common problem amongst furnace units, that can take place over the course of their lifespan. If this occurs, you can try to clean the sensor. Although, oftentimes it’s in your best interest to replace the ignitor/sensor altogether. However, it is possible for a flame sensor to go bad; but often, the sensor is not broken, in fact it is just dirty from a buildup of materials and particles in the confined area of the furnace.