Don't remember where it is to be grounded but maybe try another spot? That would be that you can say you grounded it in two different places? Maybe unsnap and resnap the two connectors again? Check the connectors for damage or something not looking correct? If still having light come on, I'd go ahead and put it back to normal to see if check engine light comes back on or not? If it don't, get with manufacturer tech support. I had an power commander 5 that was defective and tech support logged in and confirmed it. He initially said that it is very rare to have one go bad but it happened (Murphy and i), I say this to say it is possible yours may be defective?
I've grounded it in 2 different locations. One was my battery and the other was my winch ground. I've disconected it and reconnected it twice. Both with no luck. I unplugged it and the check engine light stays off