I have an 09 G8 GT that I had the Solo Performance Mach Shorty Exhaust put on, within 24 hours that P0157 code came up. I have a Diablo Predator tuner for the car, it gave me the code and by re writing the original tune the code went out, but it came back 24 hours later, I took it back in and they cleared the code and checked for leaks and the oxygen sensor, the code stayed away for 4 days then it came back. I can keep getting rid of it but it keeps coming back. Any ideas what may be going on ?