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 left hand side panel if access available.
Remove flow pipe from diverter valve and heat exchanger.

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.
Note:
If the side panel cannot be removed then the mirror can be used to check port.