I have had the P0420 code intermittently popping up over the past year or two. Earlier this year I noticed my gas mileage starting to get worse, and I am not sitting at 12/20 while it used to be at 18/27. I have replaced both upstream O2 sensors, and a read one that was showing weird voltages. This past weekend, I also started to get misfires, with the car first registering them in cylinder 1, and now in 1, 2, and 6. Along with the codes for the misfires, I am getting a P0026 code. The car doesn't have new oil, but it is still fine and at the correct level. My car's internal code reader also listed both a C0045 and a C0071 codes. I have been trying to chase the P0420 code down with no luck, and with these other codes showing up I am at a loss as to what could be causing this. I am not sure if it is one issue, or multiple issues. But I am not smart enough with cars to piece it together on my own. Also, I bought an ODB2 adapter cable for my laptop, and am using the software that came from it. What could I look at that might give me insight as to the cause of these problems? Also, I tried downloading RomRaider, but it was a lot of stuff and I am worried that I might mess something up using it. However, if it really is the best for this stuff, I'll try and figure it out. Thank you for any input any of you might add.