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F-28 is a common error for my vaillant. I have had this problem serviced at least 10 times (all under warranty). It is an ignition problem and can be connected to a number of things. I have had the starter electrodes cleaned only to later have them replaced(4 times). I just had the "panel" replaced yesterday (looks like a motherboard or electronics panel). This is the second time to have this replaced. All of these things can give you an F-28 error message. I have an atmotech pro and have had ongoing F-28 issues for two years. They are looking at venting/air issues as well to see if this is the problem. douglas
air in gas
gas rate low
could be gas valve faulty,ignition transformer,ignition cable.
if there is a spark then its your gas valve,if no spark then check continuity on harness and spark generator,also check earth to appliance