I have a 99 Lexus lx 470 and had check engine light come on once when I didn't tighten the gas cap all the way and second most recently but it not the gas cap. I had a diagnosis of bad O2 sensor which was replaced. The check engine light was off for about 3 days but has recently returned. I think that it is the cat. coverter, but it doesn't seem to be causing a problem at this point so I give.