The small arc seems to have done something.
Would you by any chance while doing up the positive terminal flashed your wrench onto the ECU.
You must always disconnect the Negative terminal first and reconnect it last, that way you cannot short the battery.
I helped another owner who had flashed to the ECU, this had overheated the ground wire which melted and shorted a few other wires inside the loom
to ground, luckily the wire doesn't go too far and he was able to repair the damage with tape.
Exactly what I did... I know it's mechanic's 101 stupid mistake. I think I got that all sorted though, I hope.