One exhaust sensor before cat or cats and one after on all years. Two versions of the single exhaust-'96-'00 Federal with one cat and Cali. '99 and '00 and Federal '99-'02 with two cats in line off the single exhaust bank. Two-cat systems used the wideband A/F sensor in front.

Makes sense now. Thanks for clarification.

Welcome to the forum. Looking forward to your participation.

Took it to the dealer because I have no time to mess with it. They said they found two torn “mass air flow sensor tubes” and that these tubes would cost $100. Anyone have any idea what tubes he may be referring to?

I have no idea what a "MAF sensor tube" is. Unless they mean the actual MAF sensor wires (the part that we typically clean when they get dirty).

Check those and see if they are torn/broken.

Don't mean to thread jack but I was trying to find an answer without posting. I did get the P0171 code previously but after cleaning the MAF sensor it went away. Changed the O2 sensors, Fuel filter, Coils and sparks too. I still get a random hesitation and misfire with little power. Have to pull over and re-start the car but no code. Suggestions?

Has anyone replaced the upstream O2 or called the fuel/air sensor while trouble shooting this issue? I am trying to do it however the plug sits on top of the transmission and I cannot figure out how to get the darned thing out without dropping the trans or lifting it off the frame.

Manual 4wd 2000 runner.

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Replacing this resolved my 0171 code...

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Actually, I had a P0171 that I was able to resolve by swapping the A/F sensor, even though a self-described "Toyota Master Tech" on here told me it could not be the cause. The FSM does list it as a cause:

I have the same rig ('99 and '00 are clones) and I think I got that clip undone through the shifter hole. I then left it unattached without further problems. Then you can just unbolt the sensor, throw it over the top, and undo the connection where it hangs down on the other side.

Mine was a combination of vacuum leaks and a dirty MAF.

I just resolved my p0171 code that popped up the other day. I spent $9 on some CRC Maf sensor cleaner and about 15 minutes and that cleared the check eng light and error code. It also ended my bad pinging (worse as the temps rise). Will be replacing ign wires with NGK, new plugs, new denso O2 sensors, and new oem pcv valve.

vacuum leak, too bad there are a billion possible places!

I'm getting this code as well. I believe it started when I replaced my valve cover gaskets. I cleaned the throttle body and IAC valve. I WAS getting a high idle as well.

Things I've Checked/Replaced:
-checked the TB gaskets.
-replaced MAF
-replaced both O2 sensors
-replaced the charcoal canister thing (forget what it's called)
-replaced a torn air boot
-checked the vacuum lines & replaced a few
-replaced throttle position sensor
-replaced fuel filter

After replacing the Throttle Position Sensor, my high idle is no longer an issue. However, the code was triggered AGAIN today!

I think it's one of 4 culprits: 1) over cleaning the throttle body and somehow it's not closing all the way anymore or 2) idle air control valve is shot or 3) the charcoal canister i bought via ebay is bad or 4) bad fuel injectors (242k miles on original).

I have seen decreased MPG and I'm getting a stronger smell of gas outside the vehicle.

I have purchased reconditioned fuel injectors (MotorWest Performance; recommended by Timmy). Bought a used IAC valve fr ebay. Guess I'll start here.

Any input?

I can provide a bit of diagnosis and thoughts on your list of culprits. A P0171 can be grouped into two large culprits, a sensor pertaining to fuel or air delivery to the engine (uncommon) or a leak in the air system causing unmetered air to enter the engine (most common).

1) Over cleaning the throttle body usually results in a clogged IAC valve when deposits slip through the holes on the bottom of the throttle body which would result in high idling, poor idle when hot on a long drive, or intermittent stalling. I've yet to hear of one throwing a P0171 code.

2) See answer 1 above.

3) The charcoal canister would not throw a lean code unless the vacuum lines themselves are leaking air, or the unit was somehow cracked or compromised. A clogged charcoal canister will normally throw EVAP codes. I would say this is unlikely.

4) Bad fuel injectors, or my top suggestion of bad fuel injector o-rings could be the culprit. Trust your nose, if you are smelling unburnt fuel, that means there's a leak downstream of the injectors. Fuel smell is never a normal thing and should be addressed immediately. A bad o-ring on the fuel rail or injectors themselves can also cause un-metered air to enter the engine and throw a code. If you are smelling a combination of un-burnt fuel AND a sulfur like smell, it's and exhaust leak between the cylinder heads and the first catalytic converter, I just took care of that on my 4Runner.

Sounds like you IAC is the only thing, I'd give it a shot. Mine was like $50 (off amazon iirc, was OEM) and took me maybe 30-40 minutes all said and done. Just don't forget the gasket.
Unless you have an 01-02. One of the small changes to the engine bay was making the throttle body and IAC integrated as one unit, and they're MUCH more expensive too replace. You could clean it, but personally I figured why not replace it if it's so cheap and at such high miles? Gonna need to be done eventually, and if it's lasted 200k so far ~~