Yea, not all fault codes are supplied in the manual, some are supplied to plumbers on training courses.
E168 usually relates to a fan fault, but can also be caused by a PCB (electronic chipboard) failure or a problem with pressure.
Check for leaks near the boiler and that the pressure gauge is holding steady.  Listen for the fan.  If it continues, a plumber should fix it for around £300.
                
 Will
                
 November 2010