It's too hot.
U comes up when the boiler is over 100°, cos it can't show 100 in the display. So U5 is 105°, U9 is 109°. That's as hot as it will get before the overheat stat trips and you'll then get E9 showing.
This could be due to air in the system, or a blockage, or a failed pump, or even a failed sensor. First things first, check the system pressure and make sure it's not on 0, if so repressurise the boiler.
If the boiler is under 2 years old call Worcester on 08457 256206 for a warranty call.
Bunslinger, May 2009
