Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Ensure radiators are open and water can circulate around the system
Attach temperature clamps to the flow pipe (30cm from boiler) and
return pipe of the primary heat exchanger
Note:
Ensure a good connection is made with the flow & return pipes
or mis-diagnosis will occur
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to fire and run for CH for 5 minutes.
Measure for temperature of the CH flow pipe and primary return pipe.
Is the primary return pipe temperature inside the boiler
higher than the flow pipe temperature?