Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
Unclip Diverter Valve head from the body and leave in a safe location
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

FOR INSTRUCTIONS ON SERVICE MODE CLICK HERE
Test 1:
Operate the appliance in maximum output mode and carry out a performance test.
Test 2:
Operate the appliance in minimum output mode and carry out a performance test.
Expect the readings to be within the parameters outlined in the manufacturers instructions.
Note: Refit diverter valve head to body after test.