Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Appliance drained and discharged
Remove the integral bypass assembly

Observation:

Inspect the inlet and outlet of the bypass valve for any restrictions, blockages or damage.
Press the centre of the bypass valve with an appropriate tool, it should move to the open position with gentle pressure and then when released it should spring back and seat in the closed position.
Are the integral bypass valve checks satisfactory?