[Ezkaton]

So scanning for codes does stuff all because it's not an OBDII thing and doesn't provide an engine light.

However I found the culprit.
If I plug in my phone to charge - BAM, instantly Engine Systems Fault.

I originally thought it was odd regarding my partner being in the car as per my original post, however after discovering this it clicked, when she's in the car we're generally going somewhere, so GPS is up on the phone and it's plugged in.

Dash cam can be plugged into the same USB thing with zero issues, but the power draw of charging a phone kicks a stink.

Anyone have any suggestions on why it might be happening and what I may be able to do to fix it?