They are hooked up in the straight pipe. Before I got straight pipes I did replace the 4 sensors (when I first got the car) but the mechanic only replaced 3 because he couldnt get to the 4th spot where the sensor went. So then it started leaving me stranded and then I got it towed to stoneys and they did the straight pipes. My boyfriend has checked and he said the sensors are still there. So could the sensors be bad or faulty and cause the problems I'm having? Is there even a way to fix this issue considering I have just straight pipes now and no cat?