Checking the ECU, AFAIK is by process of elimination, as in my earlier post; it(the case) must be grounded back to the battery; through the sheetmetal is fine. Usually; passenger-side apron to core-support to jumper to battery negative. There should also be a redundant ground strap from the rear of the passenger-side head to the firewall, but this has nothing to do with your problem....... unless both grounds are faulty!