when your check engine light is on, doesnt your engine run richer? i think it has to do with the fact that if there is really a problem that they dont want the engine to just lean out and stall so it runs super rich. Maybe its just with the o2 sensors though. I put my headers in my 99 accord v6 and the primary o2 sensor wasnt plugged in and when i should have been crusiing at 2,100rpms i was at 2,500rpms. :dunno: