Q: Hello I have 96 civic and in the process of doing a swap to a B20B . Would I have to change harness since both are obd2 ?

If the engine you are substituting is not a catalog OEM replacement, you will have to change (or modify) both the engine wiring harness and the engine computer. Engine computers are not interchangeable across engine families or even from year-to-year. The "new" vehicle configuration you are creating also has to be certifiable insofar as Federal emissions law. Please see the relevant Federal EPA Engine Swap Guidance Document. California, and a few other jurisdictions, have additional requirements and, in the case of California, these are published in the California Engine Change Guidelines. Unless you have very sophisticated test equipment and a thorough understanding of what is allowed in an engine swap, you are much better off keeping the stock engine and, if need be, simply rebuilding it. Indeed, that is the advice stated up front on the California guidelines document. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.