Bad O2 sensor will not light up a MIL on a B13...I drove with no O2 sensor for like a few days- no real gains in performance or less MPG...the sensor didn't work at all before...no MILs. OBDII cars have 2 sensors..the new ones like the QR or QG engines have 4 of em. Mike Jez 93 SE-R turbo 01 Max, w/flame thrower exhaust :) rice but nice :)