Which technician is most likely correct when dealing with a furnace that lights up but shuts off quickly?

A furnace that lights up but then shuts off quickly often has a problem with the flame sensor. The flame sensor is a safety device that detects whether or not the furnace has a flame present. If the sensor is dirty or malfunctioning, it may not correctly detect the flame, leading the system to shut down as a safety precaution. Therefore, if the technician identifies a dirty flame sensor and suggests cleaning it, that diagnosis is typically correct.

In contrast, a shorted gas valve, as suggested by the first technician, would likely result in other symptoms, such as the furnace not igniting at all or an inconsistent flow of gas, rather than lighting and then shutting off quickly after ignition. Providing the right maintenance and cleaning of the flame sensor can often resolve issues of this nature.