Mornin! New here. 2005 Grand Prix displaying ERROR MI on the DIC. Check engine light was on and I had P0030 and P0420 errors as well. I started with replacing the downstream O2 sensor, then the upstream, and finally the catalytic converter w/both gaskets. I was finally able to clear the codes without them coming back after installing the new cat and the check engine light hasn't come back on but the ERROR MI message is still displayed. Any ideas on what else I could look for? My MPG has gone to s**t and it's driving more like my wife's old 4 cylinder Vibe. No power!