Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Unclip Diverter Valve head from the body and leave in a safe position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Press and hold the service (Spanner) button for 10 seconds to put the appliance into service mode. The button will illuminate.
Test 1: Turn the CH control knob to MAXIMUM and measure the CO2 with the appliance operating at MAXIMUM output.
Test 2: Turn the CH control knob to MINIMUM and measure the CO2 with the appliance operating at MINIMUM output.
Press and hold the service button for 2 seconds to leave service mode.
Expect the readings to be within the parameters outlined in the manufacturers instructions.
Note: Refit diverter valve head to body after test.
BOTH MINIMUM AND MAXIMUM CO2 READINGS CORRECT
CO2 READINGS OUTSIDE OF TOLERANCE
BURNER DROPS OUT WHEN IN MINIMUM MODE