Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature clamps to flow pipe just below the low pressure switch and the return pipe just above the pump nut as indicated by the red arrows
Boiler CH thermostat set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the burner to ignite and the boiler to run in central heating mode and the system to heat up.
Measure the temperature differential between the flow and return pipes.
Expect a differential of approximately 25°C or less. Is the differential as expected?