b4 jumping to conclusions and speculations just pull out the o2 sensor see whats going on and go from there, he just had a new exhaust put on so possibly it was installed incorrectly or f*ed up with the new exhaust profile, pull it check out its condition, this should have been done as soon as u recognized the ecu was throwing these codes. when something goes wrong dont jump to other things, when ur car doesnt start u start with the starter, and battery u dont jump to the spark plugs, always try the simplest fixes first, and dont overlook the simplest of factors because this is most often the problem, after working on cars for many years now i have learned that much at least.