This sort of thing should be trivial to troubleshoot – or at least narrow down to eliminate anything electrical.
The problem is that almost nobody in avionics (or GA electrics) understands electronics.
The EASA B2 license syllabus is utterly banal and the electronic content (some of which I have seen) is atrociously bad, with topics written by some 5 year old kid. It is little more than a license to wire things up while following a wiring diagram. In the UK I have only ever met one guy in the GA business who actually understands electronics, avionics and the basic principles, but he seemingly spends most of his time away on bizjet AOG jobs, and unfortunately his firm is known for creative billing for non-required paperwork.
