how can i fix?

Hope your feeling lucky. Your fault code indicates that you have a problem on the PCB (Printed circuit board). These cost about £10 a hundred to produce in Taiwan, and all boiler manufacturers sell the for above £100 each. Identify your re-set switch (if I remember correctly on the Isar this is a button/hole near the selector switches) using a instrument like a ball point pen, push in and hod the re-set for 3-5 seconds. If this does not work try switching off the power to the boilr for 2-3 mins. If still no joy I'm afraid your goung to have to get a specialist in and the cheque book out. Good luck.

Dumb Plumber
April 2010