Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain the domestic hot water circuit and remove the electrical connection to the turbine
Remove the flow sensor assembly
WARNING!
There may be residue water even when DHW inlet is turned off and water drained, water could get in to the electrics when flow turbine is removed.

Observation:

Examine the flow sensor assembly, return casing and flow limiter/filter for signs of restriction/blockage or damage.
1. Return casing
2. Flow sensor
3. Flow limiter / filter