Accent Oxygen/Fuel Sensor

Just had 2002 Hyundai Accent inspected and emissions tested…also had CV halfshaft, labor to R and R, clean up and grease and install new half shaft left side and brakes and tires. Anyway about 2 weeks after spending $1200.00 for all of this the check engine light comes on. Mechanic says that codes came up for oxygen sensor and fuel sensor. I wanted to know is it possible that during some of work that was done could there be something that wasn’t adjusted and is causing these lights or anything else could have happened…I just hate to think of spending more money on getting another part and labor …that is what mechanic told me…he basically said I probably need another new part …I saw something online about oxygen sensor and emissions so I have to wonder about this