Having switched on all appears ok.fan, pilot, main burner, all start as usual. Then after a short time main burner goes off.Pilot & fan remain on, but main burner remains off.Heating pipes only barely detectable warm. pump running ok. Boiler is on gravity HW system & set for it.
check overheat stat or thermistor with multimeter or replace them (both pretty cheap) this may be why boiler goes off after a short time. if not these you may have a faulty gas valve, bit more expensive.
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Try fixfault.com to fix home heating systems.