Grant oil boiler only works on test switch

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15 year old oil fired external boiler has just stopped working. The overheat button is out but will not reset. There is no red light on the burner but it runs when I switch the test switch on. Can anyone point me in the direction of what the problem may be please? Can I leave the test switch in the on position in this emergency?
Thanks in advance.
Model is 60/70 if that makes sense.
 
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It's not overheated if it runs with the test switch - the test switch cannot override the overheat limit thermostat. Sounds more like an internal controls failure, probably a faulty zone valve
 
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I've just had a better look for anything obvious. The 2 little copper wires from the overheat reset button are loose where they go into what I guess is the thermostat. I literally touched them and the boiler is going now but the button will no reset. Any ideas on that please? Thanks
 
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Thanks for the answers. I'm not sure it has activated now. The button is out and won't even try to reset, just springs straight back out even with the boiler stone cold. But it fired up on demand last night then went off after 30 mins. its has fired up this morning from the indoor control.
 
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Then the O?Heat stat has not activated or it would not fire up at all, it is a last line of defence and closes the burner down, nothing will happen till it is re-set
 
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OK. Thanks for the info. It seems to be working on demand now but did not come on last night in -3 degrees so I guess the frost stat is not working either?
 

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