You're going to need to look into fuel control... oxygen sensor. We are going to assume your vehicle's CO reads are very low. Though this may appear to be a good thing, in terms of emissions reduction, it may not be. The catalytic converter needs a small amount of fuel for efficient emissions reduction. When a vehicle runs lean, the CAT is not able to perform its functions properly. We recommend inspecting your GMC Suburban's fuel control system and ensure the system is not running lean. Read more about what causes high NOx emissions.