Do a Scotty smoke test, keep an eye on intake valley for leaks around intake gasket. Check fuel pressure and filter too.  If those good, injectors are shot.  Most common issues. 

P0171 System Too Lean Bank 1

P0174 System Too Lean Bank 2

This particular code is triggered by an O2 sensor.

The O2 sensor measures the air:fuel ratio leaving the combustion chambers.

The O2 sensor tells the computer if it's too rich or too  lean and the computer attempts to adjust the  fuel going to the injectors.

But it throws the code because on 2 occasions either the computer wasn't able to make the high adjustment (fuel trim)  or the trim was  beyond the specified allowable limit.

The problem IS NOT the O2 sensor.

When diagnosing a lean condition as a novice, it's best to step back, take a breath, and wrap your head around what's happening.

YOU, the driver, controls how much air is being sucked into the intake manifold.

You press on the accelerator pedal. The throttle plate (butterfly valve) in the throttle body opens in proportion to the amount that you're mashing down on the pedal. The more you mash down, the more the throttle plate opens (and the more air gets sucked into the intake).

YOU CONTROL THE AIR INTAKE.

Simultaneously  the computer is using  input from several sensors, (throttle position sensor, MAF, MAP, ECT, and O2 sensors to control the amount of fuel to the  fuel injectors

The computer wants to maintain a 14.7 parts air to 1 part fuel ratio to the amount of air that YOU are demanding to be sucked into the engine.  (it's called the stoichiometric mixture).

THE COMPUTER CONTROLS THE FUEL

So this explains the lean code.

The computer is having a problem maintaining that 14:1 air to fuel ratio.

It (thinks) it's  adding all the fuel it can but something's wrong.

It could be anything from just too much unmetered  air that it can't compensate for, or an erroneous MAF signal,  to some issue in the fuel delivery area. (failing fuel pump, obstructed fuel filter, faulty fuel pressure regulator [low fuel pressure issues] or dirty/clogged injector(s),  where the fuel the computer is instructing to be added isn't being delivered to the combustion chambers.

Now before I'm done with "wrapping our heads around what's happening", YOU the driver controls the AIR INTAKE in another way.

It's YOUR job to maintain the engine so there isn't any "unmetered" air entering the intake manifold through vacuum leaks.

So, does that Autozone printout have the freeze frame data on it?

If it does, post it along with the year, make, model, engine size of your vehicle.

There's a lot of info there on the engine status when those codes were thrown.

You'd know if it happened when the engine was cold or warmed up by the ECT reading.

You'd know if it was at idle by the rpms.

What was the MAF reporting at those particular RPMs?

(A (loose) rule of thumb on a MAF's air flow rate at 500 rpm is 1 gram per second per liter of engine displacement.)

Does it look like it's a "dirty liar"?

If you had an inexpensive scanner we could examine the live data and look at what's happening with the  fuel trims.

Is the Short Term Fuel Trim high at idle but decreases as the RPMs rise?

That points toward a dirty/failed  MAF or vacuum leak.

Is the short term fuel trim high across all RPMs?

That points towards a fuel delivery issue.

In your shoes, without a scanner, I'd start by getting some MAF cleaner and cleaning the MAF sensor.

Next, grab  a can of carb cleaner, starting fluid, or the favorite flammable aerosol of your choice.

Engine running, spray it on all the vacuum lines and connections. Don't forget the brake booster, around the PCV valve hose connections on both ends, and around the bottom on the intake manifold where it mates to the engine heads. Listen for changes in idle rpms to identify areas of a vacuum leak.

If nothing jumps out, I'd check the fuel pressure.