ECU hard reset and clear?

Disconnecting the battery on any GM newer than 2016-2018 (depends on model) will not erase the codes. To erase them requires a higher level scan tool. Then you need to put the vehicle in accessory mode and clear the codes, turn it off. Wait about 5 seconds and start it. That process clears the current, pending and history codes, however it will not clear any permanent codes. The PCM has to do that after it sees a set number of drive cycles without the fault returning.