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Or do you mean the servo control valve? The fault finding guide says check it is in the CH position but does not show what position this is

I have a Vaillant 242, hot water is fine but the burner does not cut in for the heating. The relay switch clicks but does not stay made. The amber pilot light on the panel flickers faintly. Any suggestions. I have gone through the fault finding but haven't come up with anything. System pressure...