Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Selector Switch to Hot Water and Heating
Boiler CH thermostat set to MAX
Attach temperature clamps to flow and return pipes just below primary thermistor
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler and system to heat.
Does the boiler have a temperature differential of approximately 20°C or lower?
Does the boiler go to lockout again?