Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain the domestic hot water circuit
Remove flow sensor assembly from the boiler
(Flow sensor can be separated if required to three components)
WARNING!
There may be residual water even when DHW inlet is turned off and water drained, take precautions to prevent water damage to the electrics when flow turbine is removed.

Observation:

Examine the flow sensor assembly, housing, inlet filter for signs of restriction/blockage or damage.
1. Flow sensor housing
2. Flow sensor
3. Inlet filter