Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Isolate cold mains inlet to boiler, drain down primary circuit of the boiler
Remove diverter valve assembly from the boiler

Measurement:

Examine the diverter valve for blockages at the inlet port to
the diverter from the main heat exchanger.
Also examine the pipework from the main heat exchanger to the
diverter valve for blockages.
Refit parts if no blockages found.