The fuel issue is most likely related to the fuel starve issues the Z has due to the fuel tank/fuel pickup design. Because of the "saddle" design of the tank, fuel sloshes from one side to the other causing odd fuel readings. Just be happy the car didn't starve and strand you.
Now on to the AFR. Is the check engine light on? Also did you use an impact gun when doing the exhaust work (that's a big no-no)? I would suspect that if the O2 sensor is bad that you'd get a CEL. Also is the 18.8 AFR a sustained reading or just a momentary spike?
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'17 Titan Bone Stock
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