Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Ensure boiler is cool
Drain primary system water from boiler
Remove internal by-pass (see notes)

Observation:

Inspect the by-pass valve for debris, damage and/or faulty operation causing the valve to pass.
Note.
1. Take appropriate precautions regarding primary system water which may still be present in the boiler when the valve is checked or removed.
2. Make a note of the by-pass position as this is adjustable.