i have 1998 pontiac firebird with a 98 ls1 swap in it. this was done by the original owner and im the 3rd owner currently. right now the 2 codes are the 0131 and 0151 both are the front O2 sensors. i replaced both with new ones and cleared the code but they're back. the NB shows the O2s mostly in the double digits and IIRC no higher than 500mV. it went into open-loop at idle and cruise a few times but when i clear it via my handheld scanner it'll go back to closed loop and not hesitate as much at idle.

the car only has a LID and flowmaster catback exhaust. i recently bought the hp tuners and the AFR is currently set at 11.9(stock was at 11.6). that was the only adjustment i made but the codes were originally there before i made the AFR adjustment. any suggestions is greatly appreciated