Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove DHW hall sensor from flow valve
Unplug the connector from the hall sensor and place the sensor to the side for this test
Ensure the lead and connector are positioned safely
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With hall sensor disconnected, does the display now work?