Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove the Red wires (1) from the DHW sensor and measure the resistance across the terminals on the sensor.
If this test is called after fan has failed to run, resistance measured should be between 2.5 kOhms and 33 kOhms.
If this test is called after failing burner modulation with DHW at approximately 55 deg C, resistance measured should be between 2.5 kOhms and 3 kOhms.
Note:
The resistance measured will depend upon the temperature of the DHW, the colder the water the higher the resistance.
With DHW at approximately 20-25 deg C the resistance
should lie between 10-13 kohms.
If this test is called after fan has failed to run, resistance measured should be between 2.5 kOhms and 33 kOhms.
If this test is called after failing burner modulation with DHW at approximately 55 deg C, resistance measured should be between 2.5 kOhms and 3 kOhms.
Note:
The resistance measured will depend upon the temperature of the DHW, the colder the water the higher the resistance.
With DHW at approximately 20-25 deg C the resistance
should lie between 10-13 kohms.
Isolate as per TOPs P4
and refit as necessary