Hope this helps found this on another web page
The code is for a lack of system water flow.
Its measured by the temperature sensors and they can give problem trips if they are a little off tolerance.
The earlier boilers were too sensitive and later PCB software has been modified we are told.
Officially the poor flow is caused by a blocked or weak pump, dirt or air or a blockage in the system.
What the actual cause is in your case is I dont know without being on site. Clearly the "plumber" you had there is rather less than competent! I would suggest you call a boiler engineer!
The link is http://www.diynot.com/forums/viewtopic.php?t=79868
Roger Brown
January 2008