potterton puma 80 got hot water problem with central heating

as the title says i have a potterton puma 80 and its being a pain... i have got hot water (well as hot as it normally is which is not that hot and never has been) but the central heating will not stay on. when i put the central heating on the boiler stays alight for about 20 seconds then goes out (both main burner and pilot trips) i have just changed the diverter valve and the pcb (i read that it should be 1 or the other on here) but it is still happening, can anyone shine some light on what i should look for next.
thanks in advance Andy

ok just read another post about drip,drip drip... and it said isolate the cold water to the boiler and try the central heating and low and behold it has still stayed on, i have opened up the cold water to the boiler again with the heating on and it is still on !! yeh.. so my question now is none of my taps are dripping so i presume this means there is a leak somewhere. would this be on the hot or cold water ?? as i did not isolate the hot water out of the boiler.... ?????

The leak will be on the hot water side. If you isolated the boilers cold feed, you effectively shut off the hot supply to the taps.

when u said u changed diverter,did u change wax capsule as well???

combispecialist said:

when u said u changed diverter,did u change wax capsule as well???

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i changed the whole brass bit which i think is the diverter and wax capsule isn't it ??