Hello,
I just recently purchased a 99 civic ex. It started giving me problems about a week in to driving it. It has a fluctuating idle as well as a sputtering problem around 2500 rpms in any gear once warmed up as well as a loss of power. It throws a code 43. I took it to a mechanic who told me that the "engine is tricking the computer" which is causing it to run rich. It was obviously used for racing. The only modifications i'm aware of are: no a/c or radio, no cruise control, new air intake, new muffler and possible removal of catalytic converter. Any help is greatly appreciated