FYI... I have a vintage Vaillant ninety-two series boiler with an HW8600 auto igniter control. The gas valve, igniter and burner was only working intermittently at best. Noticed the plastic plug for the auto damper was loose. After closer inspection I realized the contact pins for the plug had worked loose from the soldered connections in the back the circuit board. I re-soldered the connections and now the furnace is working great !
Also had a small leak in the heat exchanger and added some Oatey stop-leak, it seems to be working. Hopefully it will last me the winter. Planning to replace furnace with usa made next year if finances allow.