On a spark ignition system, if there is a spark but no ignition of gas, what is Tech B's probable conclusion?

In a spark ignition system, the presence of a spark without ignition of gas indicates that the spark itself is functioning, but other critical components are likely not performing correctly. If the control module is faulty, it could lead to issues with sending the appropriate signals to allow gas flow or proper ignition timing, resulting in a spark that does not lead to ignition of the gas.

When the control module operates correctly, it should properly control and manage the ignition sequence, including organizing when to open the gas valve and produce a spark. If any malfunction occurs within the control module, it can disrupt this sequence, meaning while there can be a spark present, the gas may not ignite if, for instance, the module doesn’t allow for gas to flow correctly.

Other possibilities like a blocked gas line would typically prevent gas from reaching the ignition area altogether, and a malfunctioning igniter would usually result in no spark rather than a spark with no ignition. A defective thermostat would not cause a situation where there's a spark present; instead, it would likely affect the overall operation by not calling for heat. Therefore, the most logical conclusion aligns with a problem in the control module, leading to the observed symptoms.