I just bought a 2ooo GT. I test drove it and put it through the paces, including let it idle for 20+ minutes. When I went back the next day to get it, it started to miss about a mile from home. I washed it, and cranked it up to find it was misfiring at idle. You can hear it in the exhaust. I let it sit for a day, and it still misses at idle and acceleration. Its throwing codes PO156 and PO161. It does have an off road h-pipe, with one of the O2 sensors completely missing. The harness is hanging under the car. Can a missing O2 sensor cause the engine to backfire like that? How do I fix it?