Should i Try!!!?

Should i try to replace the limit stat/overheat stat and vice versa, before changing the gas valve. the boiler is a vokera excell 80 sp. i've changed the thermocouple and the pilot still won't stay on. the pressure in the heating is fine. the air pressure switch sound's ok. the fan come's on as soon as you turn on the boiler. is there a reset switch on the overheat/limit stat? if yes then where?