Flashing F21 what does this mean and can I fix it without having to call an engineer?

Gerry, December 2016

You should be able to fix it yourself, but it means actually doing something yourself, whihc yi clearly have not been able to do so far.

30 seconds with a Google search gives me (and could have given you) these gems:

http://howtomendit.com/answers.php?id=292233

http://howtomendit.com/answers.php?id=400909

http://www.justanswer.com/uk-plumbing/4fr54-ferroli-he31c-boiler-fault-f21-showing.html

XXX, January 2017