P0430 - P0402 - P1131 - Ford Mustang whats wrong?

Question

The mechanic has fixed 2 O2 sensors that were bad it was code P0131 check engine light turned off after fixing the sensors than he told me to drive the car 30 minutes and after 15 minutes through the city check engine light came on again. he said if after driving the check engine light didnt come back on than the car will need no more repairs because i need to fix this in order to smog the car i dont know what else to do he is estimating 500 dollars to fix....