I have a 2000 Chevy Prizm that was running rough and had a problem accelerating until the car had warmed up. The gas mileage has got under 20 mpg. Check engine light has started to flash when getting on the freeway and I was told to chance the upper O2 sensor (had p171 code). Now the car won't stay running and light starts flashing at start up. It has codes P300/P301 (listed 2 times). I have also changed coil packs, MAF sensor and fuel injectors. What could be the cause?