Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disconnect external controls from mains terminal strip
(two wires furthest from fuses) and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With the external controls disconnected does the boiler still show E168 fault code?
Notes:
1. On some appliances the incoming terminal strip is located differently and has been rotated through 180° from the one shown.
2. If link is fitted as in picture select "no external controls fitted".
3. E168 may be intermittent, if this is the case please use engineering judgement when choosing option.