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Have you checked to see if any snow is blocking the exhaust vent termination outdors?
I'm going to open this up to my colleagues so one of them can help. Please don't respond (even to acknowledge this post)until you get a response from us. If you respond first it will only delay your answer.
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click here for the system wiring diagram
its on page 50
Tell me if if you feel competent electronically to trouble shoot this or not, we can go from there
Tell me if you are using this to heat the house and... to make domestic hot water... or just space heating
I will post the manual again This manual
Turn the power off at the boiler, next open 3 hot water taps in the house, then stand next to the viessmann unit listening intently, first the inducer fan should start, then you should hear several clicks over a period of 45 seconds as the various relays close contacts in order power the igniter and gas valve. Measure the voltage between terminals L and N, at item 35 (the gas valve) on the printed circuit board during the entire ignition cycle,
Tell me how far the boiler gets along this ignition sequence please... and if you ever get a voltage read across terminals L and N on item 35