Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature clamps to the flow and return pipework as indicated by the red arrows
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to run in CH mode for 2-3 minutes.
Monitor the differential between the flow and return pipes.
Expect a differential of approximately 20°C or less.
Is the temperature differential as expected?