My 2.5L engine (175,000 miles) had the o2 sensor just below the manifold went (check engine light came on and I had the code checked and confirmed it was the o2 sensor) bad the other weekend. I replaced, and after a week, the new one went bad. When I removed it, it had a lot of black soot on it. I checked the spark plugs and they had some soot on them too. Any idea what the cause is and how to fix it?