boiler fails to fire up?

when first trying to fire up green light appears but no red ,after touching several parts within the casing with hands it fires. once alight ok all weekend. fails to light a week later
boiler 24cdi OF lpg ,pump is working , last week before it fired up a noise could be heard in the boiler housing but everthing was switched off, water pressure was just under 1bar

gordon tongue
September 2009