Intermittent piezo not firing up?

I found that the boiler would sometimes fire up when the circuit board was removed and the wires to the thermistor were wiggled, suggesting poor contact. So I removed the entire PCB after disconnecting all the connectors (take a digital picture first to remind you of what goes where), then use a hand lens or low power binocular microscope to view the soldered joints. I found over 10 dry joints which I re-soldered. It now works perfectly.

Teacher
December 2010