Start by getting the Diagnostics Trouble Code.

https://www.honda-acura.net/forums/s...d.php?t=179419
Click that link, and scroll down until you see the how to retrieve a CEL code.

Did you disconnect your o2 sensor in the process? I think in that year they were located on the exhaust manifold. If this is the problem, you'll need to have an o2 bung welded onto your new header. Any muffler shop should be able to do this for you fairly cheap. Good luck!