I had a bunch of maintenance work done at Chapel Hill Tire in University Mall in Chapel Hill. The following week the check engine light came on. I brought the car back in, insisted that they check what was going on for free (when they wanted to charge me), and they said after doing a computer engine analysis, codes 21 02 Sensor and 52 Knock Signal came up. The mechanics said it has to do with my fuel efficiency. The mechanic said it could be a fluke so he turned off the check engine light. Now it keeps coming on on its own, going off, coming back on. Any ideas why? Should I go to a separate mechanic? Is my car going to die if I don't?