Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove selector switch control knob
Ensure NO demands
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset the boiler / cancel pump purge as necessary.
Using a small flat blade screwdriver turn the HTG trimmer slightly - this will make
the display show the fan speed in multiples of 1000 i.e. 5.2 would be 5200rpm.
Check the value displayed corresponds to the table for the appliance you are working on.
Is the fan speed set correctly as per the table?
Note:
Leaving the trimmer for 10 seconds or greater returns the display to normal.