About a month ago, my check engine light came on as im driving down the road in my 4 door brick (243,000 miles and still moving), but i took it into the shop at my school and the instructor showed me how to read the check engine codes. It said bad oxygen sensor. so i looked over my exhaust system from sensor up, and the gasket on my header-downpipe was burned out. so i fixed that, and reset the check engine light computer. the light stayed off for about 100 miles, then came back on. no more leaks in my exhaust, and nothing obviosly wrong that i can see. I unplugged the oxygen sensor so that i can still drive it arounbd town, but its killing my gas mileage. but if i leave it plugged in, the motor bogges down so bad that my top speed is around 10, and if it idles, it dies. with the sensor unplugged, it seems to run a little rich,(obviously) but no other problems. any ideas??