Hey. I recently purchased a used Infiniti g35 2008 with 150k miles on it. The next day I bought it my check engine light came on. I checked the code and it was p0021, p0011, and p0430. The next day I brought it back to the dealer and they took it to their mechanic to get it fixed. He said that he replaced the sensors and updated the computer software. I drove home that day with the light off and when I got home the light was still off. I drove to a park later that day and the Check engine light came back on again. I scanned it for codes again but this time it said: there are no codes stored in the module. I was so confused so I brought it to AutoZone's and the same thing popped up. I ended up bringing it back to the mechanic and he tried to fix it. The same thing happened: after I got home and drove for a little bit the light came on once again. After I took it to the mechanic again and this time let him keep it for 2 days to ensure that It is 100% fixed. He drove it 81 miles and the light did not come on. I relived that the problem was fixed but after I drove 92 miles the light came on again. The same problem persists. When I read for codes it says there aren't any stored but the light stays on. The car has no driving problems that I can feel and I feel like it's just some random computer error that won't affect the car. Should I try to return it? Or is there something I can do or tell the mechanic to do that will fix this. 

I know this was kind of long but any help is appreciated thanks!