So I stored the wifes G8 before Corona. I know nothing about G8's more of a diesel guy. Anyways, just before storage she had some exhaust work done and they cut out the catalytic and straight piped it. Sounded cool and ran fine. The last day before storage I filled it with high 91 octane and stabilizer, parked it in a non heated garage and disconnected the battery leaving it on a tender. Come about a week ago I pulled it out. Connected battery and fired it up. It immediately started to lope at idle. Ran like a bag until I applied throttle. I idled it around the parking lot and it stopped lopeing. Took it for a very gentle drive and the engine light began to flash and it started shaking like a plug had fouled. Shut it off and started it and the problem repeated. Currently it will start and idle but not happily. It currently is throwing 2 codes, P2273 and P013E . I'm sure this is an O2 issue but curious if I can fix it without installing an entire exhaust system. I know nothing about these cars yet. Help!