I changed the rear after the cat. Convertor oyxgen sensor. I couldnt get the car started. So I put the old one back on. Still can't get the car started. I disconnected the negative terminal to clear the codes. I had the car running to heat the exhaust in hopes of trying to loosen the nut easier. When the nut broke loose the car stopped running. Didn't think this was a problem so I swithed out the sensor. I put the old one back on. Still cant get the car started.