my Baxi 105e wont fire up?

The fan runs ,it sparks but then goe to lockout with red ign fail light - have reset 10 times but wont restart -volts to solenoid OK but one side of the double ? coil looks opn circuit -(can you change the coils without buying a new agas valve ?) what does the small solenoid with a plastic pipe coming out of it do ?)

chris
April 2013