Air/Fuel Mix Too Rich

Vehicle: 2003 Ford Explorer Sport Trac 4x4. Mileage: 36,000. I recently scanned my explorer and the code said "Air/Fuel mix too rich". I know that this problem is commonly caused by a faulty O2 sensor. But I was wondering if any other components - like for example, the EGR - can cause the same problem. If so, is there any way to clean/repair an O2 sensor or an EGR valve? Bear in mind that my engine is recently developing pings and knocks when overdrive engages. Can this be caused by the faulty O2 sensor? Also, the exhaust smells like rotten eggs and at idle, the engine is not steady. Finally, whenever I fill up my gas tank, the engine starts but inmediately dies. I have to restart a couple of times.
P.S. The truck was flodded, so it wouldn't be strange if any if the mentioned parts are failing (given the not so extensive mileage).
Thanks a lot.