I had a check engine light that would sometimes not come on for a few weeks, and sometimes during a long drive. The dealer said to check the gas cap, and after several visits to test and reset the light, I bought a replacement cap. That didn't fix it. This went on for about a year or so until a dealer tech checked a little deeper and found that two hoses leading to an evap canister had been swapped when it was replaced. He corrected that and it hasn't recurred (been almost two years).