· Demon Spawn
he just about nailed it, damn arathol your trigger is quicker than mine recently.O2 sensor malfunctioning
MAF sensor dirty or malfunctioning
check for vacuum and exhaust leaks
PCV system malfunctioning / valve stuck open
check fuel pressure, weak fuel pump / clogged filter
If there is nothing immediately apparent you might want to have it checked using a professional type diagnostic machine. Its easier than throwing parts at it.
to easily check for vacuum leaks, outside of the obvious ripped or disconnected hose is to get you a spray bottle with water/dish soap solution and spray it on the hoses while the car is running any spot you see bubbles has a vacuum leak, flammable carb or throttle body cleaner will work too (just gotta be more careful since it is flammable and your under a car with the engine on) if you do this method instead of bubbles if you hit a vacuum leak the engine will rev. another thing I would check while under there is the air intake hose, check the ribbed part well for any minor cracks this can throw lean codes as it is allowing in unmetered air behind where the MAF sensor picks up the air volume, so the fuel mix gets thrown off but the car wont know why, just that the readings its getting are correct from the sensors but it has more air in the mix than it should. these boot do wear out, it is rare but on these cars the motor mounts failing is a big issue and when this happens it allows the engine to buck around under the hood when your driving moving and stretching everything and one of the first things to go is usually that air boot as its rubber and rips if it gets moved around violently a lot like a bucking engine does.
check the mounts while your at this too. one is extremely easy to check, its the passenger side mount it is right under the coolant tank, look at the rubber if it looks like a deflated tire or you poke it and can see rips or the fluid inside the mount its bad or if you see what looks like oil leaking under that mount beside the engine that mount is bad, if you find that mount bad replace it and the drivers rear at the same time (at minimum) the other 2 may not need changing (I had an 08 and 11 3 both needed these 2 but the other 2 where fine still) because as the passenger side one wears out it puts all the strain on the driver side one and causes it to wear out faster. the other 2 mounts are not really motor mounts but tranny mounts (2 on the auto 1 on the manual non speed cars) both the engine mounts can easily be changed in the drive way with jacks jack stands a block of wood and hand tools. the passenger mount is called mount #3 the drivers is mount #1. another way to check for bad mounts is to start the car and have a friend sit in the driver seat watch the engine as it idles a minute note any normal movement (as they will move slightly) then have him give it gas, hard to 2500 or so rpm is usually sufficient, and watch the engine you will see it lift up and set back down as he lets off the gas if you see this the mounts need changing other symptoms include leaking fluid or stains near the passenger mount (liquid filled other 4 are solid) hard or late shifting in autos and noticeably harder time going from gear to gear (especially 1-2 or 2-3) in manuals.
this video shows you what bad mounts look like:
this shows where the mounts are (note mount 1 is UNDER the car and can be accessed back near the drivers side firewall. and no 4 is on top of the transmission under the battery tray, #2 isn't shown here but it is a big rectangular plate under the tranny with a big piece of square rubber between the plate and a cross member that it bolts to)
why did I just go into all that you might be wondering? well these mounts are a known issue on these models 13 and back non skyactiv. and when they fail they start causing all kinds of other issues possibly one like you are having where you are inexplicably throwing a lean code.
outside of this, like arathol said its kinda a toss up after checking the vacuum hoses for leaks or one that has come loose as the lean code you have is a generic code with many many things possible to fix it.
how does the car drive? any other symptoms of a problem other than the check engine light? rough, high, or erratic idle, odd shifting out of nowhere, bucking or having the rpms drop then pick back up (engine stumble)? anything else you can give us to help diagnose will help us help you