Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Turn CH Temperature to 70°C
Attach temperature probes to flow and return pipework close to the boiler
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

On/Off switch on.
Allow fan to run and boiler to fire and allow system to reach a reasonable operating temperature.
Measure the temperature difference between flow and return.
Expected differential approx. 11 to 20 °C
Click on your observation:
1. DIFFERENTIAL OK BUT EXCESSIVE HEAT
2.DIFFERENTIAL NOT OK NOISE LOCAL BOILING
3.DIFFERENTIAL OK SYSTEM PROBLEM