Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove all external control wiring connections and make safe
Ensure NO demands are present
Note.
You may encounter wire links on some connections
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the central heating demand (radiator symbol) still flash ?