worcetsr junoir 28I gas valve

[efficient]

Hi guys, Called out to a worcester junior going into lock out, Got their, reset, fired it up running fine, checked inlet pressure, combustion everything ok. Check the resistance on the gas valve coils and the top one is over 400ohms which indicates a fault. But how is the boiler working ok? I'd like to know how do the valves operate, whats the job of the first valve?
Cheers Andy