Thanks for the advice. My concern is that I can’t seem to reset the tcu. I tried disconnecting the negative battery terminal, and separately I tried removing the backup and ecu fuses. In both cases, when I turned the car back on the D4 light remained stuck on. I saw the flowchart for diagnosis that was posted in that thread but I’m not sure I can follow it ( I don’t really know what “test harness ‘A'” is), unless its simply a matter of checking voltage at a certain connector pin. I’m hesitant to just replace the tcu, I don’t want to just keep throwing parts at the issue until it goes away, but if theres reason to believe that that’s the likely culprit then I suppose I have no option. Thanks again for all the help!