1999 Chevy Silverado codes 171 174

First, you need to check the engine for any vacuum leaks.You can do this with the engine running.Spray some carb cleaner around the intake manifold gaskets and pay close attention for a change in idle quality when doing this.If it changes,you have a leak and should replace the gaskets.Be very careful not to spray any hot engine parts!This should be checked when the engine is first started after sitting overnight.