I have a 94 Dodge Shadow ES. The O2 sensor went out last April. I got it replaced and now it is giving me another 51 code and my gas mileage went from 30MPG to @ 21MPG. Is there something that could cause the sensor to go bad so soon? I'm going to replace it again but don't want to have to do this every 5 to 6 months. What are the things I should consider that can cause premature failure?