my thermal coupling lights up, when the timer clicks on?

what kind of boiler?

Sounds like not a thermocouple but you have a automatic pilot light which ignites but no main burner???

Could be gas valve fault not opening solenoid 2 allowing gas to main burner, could just be recognition of pilot not being found so boiler does not think pilot has lit.
Does it keep clicking/ignition at pilot trying to light although its lit already????

q
June 2009