i changed the stock y-pipe to a catless 2 weeks ago, ever since then, after about 10 minutes of driving, the check engine light comes on. i'll turn off the car, start, no light, about 10 minutes later, light. i had my mechanic check it out but he couldn't get a code cause it doesn't come on automatically. any ideas? sensors maybe? previous owner said he changed the o2's already. also changed the maf ?? 
