this is just a thought to try
I have have this fault F28 & F29 (recently )my book says gas problem but the Vaillant web site says to check your condensate pipe has not frozen up, i went out and separated the external pipe,it was frozen and then allowed the trapped water to run out to ground (a couple of gallons came out) the boiler then fired up will leave like this till weather improves then refit pipe
wirecutterman, December 2010