on my 2011 3/4 silverado hd, I have a yellow engine light on. The codes are 174 and 496. MAP and o2 sensor. The only sysmptoms I'm getting are when the fuel cap is removed ther is a vacuum whoosh. The the vehicle is hard to start. Sounds like the evap purge valve. Not sure but never had one cause a vacum. Ive seen pressure, so i guess it could go the other way as well. help
2011 Chevy Silverado HD 3/4 ton