Boiler won't start

hi all, have Dunkirk boiler with radiator heat in our house, when i turn the thermostat on, the auto flue rotates to 'open' but the burners don't fire up; the usual progression is:
turn on thermostat flue rotates to open 5-10 seconds later unit starts
have not touched any wiring or anything, and have tried a different thermostat with the same results. cound the gas control unit have gone bad???
any input appreciated pax, h2
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My guess would be a problem with the vent damper switch or connectors. But it helps to know how to use a voltmeter to tell if the voltage is making it through the loop to the gas valve.
My original vent damper on my steam boiler would not reopen when hot (prompting previous owner to set pressuretrol way too high), but would open and fire when I flipped its manual switch. My current vent damper occasionally opens fully (green LED on actuator), but fails to fire. I suspect a faulty plug (I should swap it for the new one that came with the new damper).
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