For the past 4-5 months, I've been getting a CEL that comes and goes. I used to think it was a loose sensor, but then I noticed that every time I bought Shell or Chevron gas, the CEL would go away. All the other brands of gas would give me a CEL when my tank was about half full of that gas. Does this sound reasonable? Or am I just imagining things? I used to be able to buy gas from any station (93 octane - Go Texas!) and not have a problem. Now I don't really mind having to find a Shell or Chevron to buy gas, but I'm worried that there is a bigger problem with my car. Any thoughts would be greatly appreciated! I have an 2002 5 Spd with intake and exhaust if that makes any difference.