Set Appliance State:

WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

This code should only be displayed if the diverter valve has been set into service mode via the installation mode.
Press the restart button under the display (right hand button)
Is the code still present?
dU CODE CLEARED - RETEST BOILER
dU STUCK ON DISPLAY