I have no codes I will check everything you all told me too when I get off and hopefully it will be a simple fix. But no there is no code or engine light on at all just didn't even know where to start to find the problem. But is there a way that the previous owner could have disable it maybe cause they never hauled anything on it or didn't like the noise from it. The Denali was a vacation vehicle I got from a family and that's all they used it for the husband said he never towed with it. But on he other hand it pulls great and really smooth to not have the shocks aired up. I really appreciate all the info