P0171 & P0174 Error Codes

Partially restricted fuel injectors will cause too lean condition. Good fuel injector cleaner. If not you have vacum leak for sure. Usually intake gasket.

I know this is an old post, but I found the solution for my 01' Yukon P0171 and P0174. I have changed all the vacuum line and the PCV, throtle body gasket, MAF, plugs wires, fuel pressure reg. All was changed and the codes always came back.

I finally noticed the purge valve on the manifold was constantly purging. I replaced the purge valve abnd code has not been back. It was $40.

For one thing, cleaning a MAF doesn't always work. Sometimes they just need to be replaced. Especially depending on what kind of cleaner you use on them, the cleaner can fry them. Also, both banks being lean and running worse when cold more points to a vacuum leak. Check around for any broken or loose vacuum hoses, and when the engine is cold and running poorly, you can use either carb cleaner or a propane wand to check for an intake leak. If the engine runs better for a couple seconds, you've found the leak.