I have a Glow Worm Fuelsaver Mk2 model 40B. The pilot can be ignited and heating will start without difficulty.
After a few minutes, the pilot lights off. The overheat stat was thought to be faulty and was replaced. But it might not have been the exact same part number (the plumber had to replace the female plugs by male ones) and the fault persisted.
The plumber has also checked for airleaks.
When the overheat stat is disconnected, the fault persists. The pump stays on after the light is off.

Any help on this would be greatly appreciated.

joel, January 2013

Replacement Overstat was not exactly as original but more sensitive.
damaged the replacement thermocouple so light off.
new thermoscouple with correct overstat.
sorted

le bars, January 2013

Thank you for answering. Here is the detailed sequence of events:
sunday 06/01; return from holiday. pilot off. no reignition possible.
monday 07/01: called a registered plumber who changed the thermocouple, cleaned the pilot flame. he believed the overheat stat was faulty and disconnected it. Told us he was ordering a new part and to leave the heating not on at all time. The boiler was programmed to heat twice a day and worked fine.
friday 11/01: the boiler was still running fine without overheat stat .the plumber installed the new part. From there on, the pilot light goes off a a few minutes after ignition and can be reignited.
saturday 12/01: the plumber checks for air locks in the pipes but does not find any thing. He thinks that the new part is not exactly the same as the old one and is too sensitive. He disconnects it.
From there on, the pilot light goes off after a few minutes, with the overheat stat disconnected.

joel, January 2013

have u replaced the thermocouple. check and clean the pilot flame to make sure its getting a good constant flame onto the termocouple. if after this it persist then i could only say its the gas valve as you said the problem still persist with the overheat stat being bypassed

kash, January 2013