I responded to an urgent "no heat" service call where the thermostat was on and the blower fan was running, but the air from the vents was cold. During the diagnosis, I observed that the hot surface ignitor in the furnace was failing to glow due to a hairline crack that had broken its electrical continuity. I replaced the faulty ignitor with a new, durable silicon nitride ignitor from my truck stock, and the system fired up perfectly on the first try. The homeowner was very pleased with the quick diagnosis and happy to have their heat restored in under an hour.