Hi Stuart

My twopennorth...

ECUs and electrics are susceptible to all sorts of
problems due to the harsh environment they are subjected to in automotive use.

To say that they do not "wear out" is not technically correct therefore. Even power-cycling can cause circuit failure over a period of time.

Stand your ground. If you can't get a good automotive tech to back you up then find a good electronics man if you can.

Good luck!

Steve