Well, fixed. I thought I'd take the unit apart again and check continuity between the PCB connector and the OBD connector - all fine.
For those who haven't looked inside one of these, there are two PCBs connected by a 6x2 0.1" header. As I was reassembling and preparing to pop over to GM's, I noticed one of these headers had detached from the PCB (
click for full size):
You can see the footprint the lead left in the solder, so it was clearly a dry joint and I'm surprised it ever worked at all to be quite honest. Terrible manufacturing, but what can one expect for £75 odd.
Resoldered all 12 connections on that header, and voila! All working again. Something to suspect if this ever happens to one of you
