sigley0527

2000 4runner Limited/V6 3.4L.

My truck has been throwing this code for quite some time now. My good friend is also a mechanic at a private shop. We have been troubleshooting this for 2 weeks now, with no solution. We went through the entire OBDII manual and can not pin point the problem.

Here is the trial and error run we have done...

-I replaced A/F ratio sensor (from Toyota), cleaned MAF sensor, and replaced air filter. OBDII computer says both O2 sensors and MAF sensors are running perfectly.

-We smoked out entire motor to look for vacuume leak. NO LEAKS.

-Inspected entire exhaust system for leaks. NO LEAKS.

-TPS sensor is clean and working properly.

-Considering there are no symptoms and difference in driving performace we feel we can rule out a problem that has to do with the fuel system (fuel injectors, fuel filter, etc.)

After all these repairs/tests we cleared the codes and reset the ECU...and put the truck on the dyno and followed the 4runner guide to reset ECU (double trip). (Drive 50mph 3-5min...idle 3-5 min...etc...etc..) The light keeps coming back on after 30 or so miles.

We just cant figure this out and I'm really frustrated. I must include the fact that my truck runs perfectly even with the light on; no hesitation, bogging, rough idle, etc. I really dont want to have to take it to the dealer for them to diagnose the problem and try to jerk me around. I have too many resources of my own to fix my truck.

My friend (mechanic) thinks the P0171 code might be because the Cat is starting to go...normally he wouldn't think this but through the process of elimination we have done this is his only suggestion...But doesn't a bad cat throw a code of its own??? His only other suggestion to me was to just continue to drive my truck with the Check Engine Light on, and wait for symptoms to arise so we can have more evidence as to where this problem is coming from.

Any suggestions would be greatly appreciated. I take good care of my truck. There is only 100k miles on it and I never plan on selling it so I want it in the best condition possible so it could have a long life. THANKS!

Nelsonmd

Yeah, the CAT code is P0420, lots of us drive with it. I refuse to pay to fix it.

Have you read through the factory service manual? if you could get your hands on a more advanced OBD2 reader (like they will have at the dealer), then you might be able to get more detail. Even a scan gauge can read your A/F ratio, though perhaps not super accurately.

xxxtreme22r

P0171 OBD-II Trouble Code = System Too Lean (Bank 1) A bad cat would not cause engine to run lean. As this code is thrown from the up stream o2 sensor not the downstream sensor.

If your positive you have no vacuum leaks that would cause a lean condition, have to tested the MAF? Also check your intake hoses for crackes etc, specifically the one(s) between the MAF and throttle body.

BajaRunner

Most likely upstream O2 sensor like extreme said.

You'll have a efficient running truck after all this though... hah.

xxxtreme22r

Didn't say it was the upstream sensor especially since (well I assume anyway) he said he changed out the A/F sensor (which to me means upstream sensor). Just mean the sensor is what throws that code as it's the one which reads rich or lean.

Usually when I see this it's accompanied by a Random Misfire code as well. Which number I don't remember offhand.

Any chance you used a bosch and not a denso o2 sensor?

stellarbluepearl

You mentioned you cleaned the MAF sensor. Did you clean the two hot wires inside the sensor also? I had the same code and dropped a lot of money into it. I cleaned the bulb looking thing on the MAF sensor and still had the P0171 code. Went back in and cleaned the two small wires inside of the sensor and that fixed the problem. To do this, you will have to completely remove the MAF sensor and look down inside the sensor. Good luck.

TheDurk

You've been through the entire cause list in my '99 FSM except one: the engine coolant temp sensor. If you can find an OBD2 unit that does live scanning, I'd check that Temp. readouts look appropriate as engine warms up just to cover that base. It would also be nice to be able to swap in a known good MAF sensor for a test. That MAF goes on a lot of Toyotas. The MAF can degrade enough to cause problems without throwing a fail code.

Also, clean your throttle body if it needs it. It can screw up lots of things and Toyota doesn't include it very often in the cause lists.

mwc0404

My 99 4Runner is also throwing the po171 code. The cheapo scanner at autozone calls for " Air/fuel mixture system lean bank 1 (Cylinder number 1 denotes bank 1), Probable cause: Low Fuel Pressure, Faulty MAF/VAF sensor, Cylinder misfire condition, Faulty ho2/AF sensor, large vacuum leak.
Ive been working on this for the last couple of weeks, Ive cleaned my MAF sensor, I changed the plugs last week, checked for vacuum leaks and changed 5 dry rotted vac lines and the dryrotted gromet with new pcv valve, the only thing left is to change the fuel filter and test the o2 sensor to make sure its working correctly.
Im going to beat this code but if any body has ideas or has also fixed this, Im open to ideas!!!

Guyechka

Pressure regulator?

I would have said filter, too. Always a good idea to change out a filter every now and again.

Roadtripr

I went through this last year. Truck ran fine but gave the same P0171 MIL.

Tried a new O2 or A/F sensor and the light came back on.
New fuel filter - no luck
Looked for leaks - found none.


Mechanic measured the MAF and it seemed fine, but since all other stuff failed we tried a new one. Ding Ding! No longer getting code and truck was a good bit quicker, the slight hesitation that had slowly creeped up was gone. I didn't notice I had it till I got the new sensor.

Try a fresh MAF and see what happens

westjohns yota

if it doesnt run rough, hot and has its normal power, unhook the check engine light, and keep driving lol, im close to alabama, we redneck stuff all the time

Roadtripr

And risk eventually killing the catalytic converter cause the truck is not running efficiently?

allclock

May try plugs and wires.
I think after a number of misses it will throw that code.
Unburned fuel and combustion reads as lean due to high levels of o2 available.

I think I had that

99Tacoma4x4

Roadtripr,
I have a 01 4Runner from California (seems to matter on some stuff from what I've read) I have same code P0171 bank 1 too lean, when I rev engine in park it boggs or has a hesitation then clears up. When driving there is no hesitation. Did yours do this? I've read post after post and I'm going crazy trying to figure this out. I've read that it's MAF, AF sensor, Then I've been told TPS which does not go with this code??

Mike Ricotta

If you have tried everything else and are out of ideas clean iac valve by spraying carb cleaner in the hole in throttle body before the blade. Keep filling it with carb cleaner 2 or 3 times. Let sit for 30 min and use carb cleaner to force carbon buildup out the ports behind throttle blade. Got nothing to lose....it’s baisically free.
if that don’t work used compressed air. Took weeks to diagnose but it fixed my problem. Also if u have k&n get rid of it

Malcolm99

Pretty old post I hope he got it sorted by now, but thats not good advise, all that carb cleaner would end up in one cylinder and eventually into the crank case through the vacuum line on the bottom of the iac, maybe pull that vacuum line and use throttle body cleaner/lubricant and flush it out that way, but you really need to remove the TB and IAC and use a brush and TB cleaner to clean an iac, they don't hang up very often and would cause high idle or very low idle, and can be tested for range using 12v jumper and ground.