A while back I had a misfire as the coil pack stopped working. I had to drive it home on a trip and imagine it mixed my oil with the unused fuel. I have finally had the time to fix the vehicle and take some time to look it over. I have changed the oil and filter when I got the P0172 "System too Rich Bank 1". I continued to receive this code and changed the upstream O2 sensor; I got an AC Delco that is required and put it in. Now, I am receiving the same P0172 but also a P0142 "O2 Sensor Circuit No Activity Detected Bank 1 Sensor 2". I'm wondering if this is saying the downstream needs to be changed now? If it helps, these are under Pending and not Stored yet. I did purchase the downstream AC Delco O2 sensor when I got the upstream. I was just lazy and a tad hungover from the mother in-laws birthday when I changed the upstream. Any advice would be greatly appreciated.