Gonbigrock

Help please! I replaced motor in maxima and can’t get it to run for nothing. It’s a used motor with 67,000 mi. The old motor had a blown head gasket and was causing motor to over heat. Otherwise the car ran great! Even with 249,474 mi on it. I would appreciate any advice on what I have done wrong? Can’t get it to idle for long before she shuts down. Starts great increases a few hundred rpms and dies. Getting a 2123 $ 2128 code I’m fairly sure that there’s nothing wrong with the peddle itself. Is there a process for figuring out what and where to look and what to do to determine what is going??