Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Selector Switch to Hot Water and Heating
Use the FGA temperature clamps to measure the temperature of the bypass pipe (A in diagram) and the flow pipe just after the isolation valve (B in diagram).
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to fire and CH to run.
Compare the temperature of the bypass tube
and the flow pipe.
Does the bypass tube get hotter than the flow pipe?