baxi bermuda 551 cat.1n-boiler unit will not fire gc no.4407748?

Is the pilot lite?
with heating / or hot water turned on, is there 240v ac to gas valve, you will have to partly remove the thermostat first to gain access to electrical connectors. (i normally turn the thermostat of then on again and listen for a clonk from the gas valve solinoid, befor i take it to bits, if you hear the clonk, then its a safe bet you will need a new gas valve) but if you just hear slight tick and you have 240v ac to solinoid then its the soilinoid at fault

Baggs
December 2007