Originally Posted by MrFatbooty
ANYWAY....
The reason you got an O2 sensor efficiency code is because the rear O2 sensor is there specifically to see if the cat is doing its job. It compares its readings with the front sensor and within the ECU there's a range of differences which indicate you're actually running a cat. Go outside the range and you get a code. If you run a straight pipe or gutted cat, that will obviously put you outside the range.
that gets me thinking would there be something you could do like idk...put a resistor on the wire to the oxygen sensor to trick the ecu into thinking that car is running in the propper range?