Good day I am pulling my hair out on this one. It is my daughters car 2005 Pontiac Grandprix it has 199000 miles on it. Started to miss and had a p0171 code and p0300 code. Plugs had never been replaced so wee replaced plugs and wires also replaced both 02 sensors as a maintaince item. Checked and cleaned the maf sensor. Before replacing the post 02 it was slow but within spec. after replacing all components listed the 0171 code is gone but still have the 0300 code and post 02 is at .89 volts. also the short term fuel trim is at 99% on the post cat 02 pre cat fuel trim is fluctuating between +6 and 0. checked for vacuum leaks with carb cleaner nothing apparent. I am going to put the original 02 sensor in after the cat to see if that changes anything. might have gotten a bad sensor.any ideas would be great. Car was running great then just started this problem.