friend's si can't pass smog test

the car is a 91 civic hb si. it can't pass the smog check.. the cat converter is stock and just replaced 2 years ago.. could it be the o2 sensor? it doesnt throw a code 1 though.. so my friend disconnected other wires from the ecu and it threw codes.. but when he disconnected the o2 sensor wire it didn't throw any codes. shouldn't it throw a code 1 if the o2 sensor is disconnected?

I dont believe so. I know it'll throw a code if its broke but if you just pull it out of the CAT (still connected)...it will still work but will give an inaccurate reading making your car run bad. On the other hand, a 2 year old CAT could be bad too. Try borrowing one from someone and get it retested.

b4 you bother tryin parts out....you need to know what the smog readings are so you at least have a smaller area to look at....any possibility of scanning the smog sheet? do you know where it failed? too rich? too lean? exh leak possibly? hell scan the sheet and ill see if i can point you in the right direction.