Vaillant Ecotec Plus 937

14 May 2016
Reaction score
United Kingdom
After replacing the o-rings on the storage tank pump due to constant leaking, the boiler is now showing fault codes F.12 and F.91 which relates to the storage tank charging sensor. I have replaced the NTC-sensor for the CH flow/ CH return (193592) and the NTC-sensor for the DHW outlet pipe (103429) but to no avail.

One mishap that did happen due to the incorrect o-rings being fitted on the storage tank pump was the failure of the connections once the water pressure had risen. Whether water has penetrated anywhere I'm not sure. But once the correct o-rings were fitted the boiler fired up for about 5 minutes before displaying the error codes above.

Any advice greatly appreciated.
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