After lighting the pilot (which does not always light first time, which it used to) & releasing teh gas knob, the fire ignites. After a period of time which is always vaiable, the flames go out as well, noting that the pilot has also gone out. I have temporarily by passed the fan pressure valve & provided a electrical direct feed to the gas solenoid - it still has this problem. The pilot is a double burner & burns with a blue flame. When it does this, the pilot will relight & the fire fires up again but will shut down again after a period of time. I have changed the electrical solenoid that works the gas valve, still does it. Is it the main gas valve (a GH 379) or the thermocoupler that is at fault? any one help!!
It'll be the thermocouple at fault and the pilot injector may need a good clean - entering CORGI territory - not sure about your temporary by-pass of the air pressure switch, how temporary was this ? a few minutes.
Get a heating engineer out, probably due it's annual gas service anyway ?
Thanks "htg engineer". The bypass was only for about 20mins while I waited for the problem to occur (it is now removed). The thermocouple is approx 6 cm in height & is obviously heated by the side flame of the pilot. From teh thermocouple & when it screws into teh back of the gas valve, there is a brass connection (soldered to the side) & a white "connection" from the centre of the thermocouple to the centre of the rear connector. I have had a look on the web & you can buy an unversial type thermocouple - is this ok? The Trident company have gone out of business!!