While sitting idle my vehicle suddenly went into reduced engine power mode. i was able to get over and pull off the road and shut off the car. 5 minutes later the car started like there was no problem. over the next few days the rep warning would show up and my car would suddenly lose power. i was crossing the road during one incident. however, when i would shut off the car and restart the check engine light would go away as would the rep warning. after a quick review online it is easy to find hundreds of people who have a similar issue. gm will only diagnose and if the light is off when you get there there is nothing for them to do except take your money and say good luck.