The lean codes from both banks are coming from the O2 sensors. These sensors weaken over time and report a lean condition to the computer. The PCM tries to compensate by richening up the mixture. Result: Gas mileage drops and sometimes unburnt fuel makes it to the cats causing a sulfer or rotten egg smell. I think 60K is about typical mileage life for O2 sensors. The uppers seems to go sooner/more often than the lower ones.
