I installed a new battery and the engine is now throwing - P0171 -> P0101 -> P0138.

I had it scanned it at AutoZone:

- P0171 -System too Lean (A/F Lean, Bank 1)

Secondary DTCs

- P0101 - Mass or Volume Air Flow Circuit Range/Performance
- P0138 - Oxygen Sensor Circuit High Voltage Bank 1 Sensor 2

I replaced my MAF and cleaned the throttle body. Any advice on what my next steps should be? Based on what I've read it could be a whole slew of things - O2 sensor, fuel injectors, manifold leak, or other vacuum leaks.

strange it would coincide with the battery change. did you clear the codes/....which ones came back?

I did. The same codes all came back.

Given you changed the battery, I wonder if you could have knocked a vacuum hose loose or cracked one. Also, I would look for loose/cracked ducting/clamps on the air intake, particularly from the MAF to the throttle. I would do a visual check and, if nothing observed, spray brake clean on those areas with the engine running - surge in rpm indicating leak. Brake clean is flammable so be careful.

I'd recommend checking for any vacuum leaks in the intake system. Those could be causing the lean condition (P0171) based on the symptoms. Check out the hoses, gaskets, and fittings for any wear or disconnection.

Next, give the O2 sensor wiring and connections a once-over. Make sure there's no damage or loose connections. Since you already swapped out the MAF, the P0101 might be tied to wiring or connectors too.

If the issue persists, check the fuel injectors for proper functioning. A clogged or malfunctioning injector could be causing the lean condition.

Is your 4runner running ok meaning is there any problems with starting and in it's driving?

I have found many time these little check engine lights seem to just pop up and i feel it's mostly really nothing, just get a small code scanner i bought one at Walmart cost $19 and i remove the codes, every time my gas cap is not seated tight and those click click click things don't make me feel it's getting seated well.

The check engine light comes on also the anti skid light flashs, this is something Toyota has programmed to send people to their Toyota stealer dealer what does a un tight gas cap have to do with my anti skid system.

Auto Zone has cost more drivers more unneeded money, sure they scan for free and they always find codes to have you replace many unneeded parts.

If you remove the check engine lights codes, and your 4runner is working ok keep on trucking and save your money for something it really is needed.