2009 Chevy aveo codes p0171 p2270 p0599

Hey guys I'm new here and I wanted a second input or more insight on this .. I've changed the valve cover gasket just recently oil was leaking into the spark plug really bad.. Had got that situated but before I continue this car been had these codes just thought fixing that would correct it . well my question to you guys what is causing my car to run lean and that other codes are for the thermostat and o2 sensor . I haven't checked the intake hoses or filter yet .. Somehow there's too much air getting in there making the engine stall out when idle .. Starts up just perfect.. Doubt changing spark plugs will fix this but I will knock it out the way .also more general info is I also hear a hiss sound coming from the cover the pcv valve I believe ??? Just changed the gasket didn't really notice any cracks in the valve cover but could unmetered air enter through there ? I'm concerned about the lean code the thermostat will be changed and the o2 sensor is being triggered by that other code I reckon . any help is appreciated feel free to correct me any where necessary