I just installed some longtube headers on mt 96 Z28 A4. I also put on new plugs, wires,LT4 knock module, O2 sims and a coolant temp sensor. My SES light is constantly on or flashing so I scaned it and found.

P0154 No activity detected in O2 sensor
P0300 Random mulitple cylinder misfire
P0400 Exhaust gas recirculator flow malfunction

Any suggestions on how to fix these problems would be helpful.

 

The o2 sensor code is for your bank 2 sensor 1. That one should be your passanger side in the header (I'm pretty sure that sensor 2 is by the cats). Perhaps the sim is shorting out? I had an o2 sensor wire fall out of its place and short causing all sorts of trouble. Stupid wires.
Check your plug wires for the misfire code, those little bastards burn easy. If you have any heat shields, make sure you check under them, you can still burn a wire even though the heat shield looks fine. Stupid heat shields.
I'm bitter because I've been there.
Good luck I hope some of that helps.

 

The P0300 can be hard to diagnose. It could be as simple as a burnt plug wire or fouled plug, or it could be the opti or anything in the ignition system. Since you just put in headers, I'd recheck all wires. The P0400 is the EGR valve. The headers have reduced the back pressure and this causes the EGR valve to not function properly. A lot of people have the EGR programmed out of the pcm and this solves the problem.