Originally Posted by HtownBlue

P0171 and P0174 I know this is long, but if you've had these codes, please read the entire post as it will explain EXACTLY what and why which will be important to tell the mechanice you work with.

If you want to know the solution, simply scroll down. What follows is an explanation of what I discovered. Let me start by telling you guys what I encountered. P0171 and P0174 are one set of codes that set off the Check Eng. and ECT and one other light on the dash I cannot recall at this exact moment...whatever.....engine detects these codes and sets off the lights.

Now what are these codes? Lean codes in bank 1 sensor 1 and bank 2 sensor 1. In layman's terms, the computer sees too much air in the system for the proper fuel air ratio and tells the injectors to stay open longer and add more fuel so as to balance out the ratio.

Now many things can play into this.
The Basics
Combustion engines operate by burning an air/fuel mixture of about 14.7 to 1—14.7 parts air to 1 part fuel. When the air ratio goes below 14.7 parts, this is called a "rich" mixture. If the air rises above 14.7 parts, it is called a "lean" mixture.

Rich Mixture = Too much fuel, not enough air
Lean Mixture = Too much air, not enough fuel

To keep the engine running properly, the Engine Control Module measures the oxygen content in the exhaust with oxygen sensors and makes adjustments to the mixture by injecting more or less fuel.

The control module operates within specific parameters and under normal conditions, it will make minor adjustments to the air/fuel mixture. When these adjustments become too large, a fault code is set. When the P0171/P0174 code sets, the oxygen sensors are detecting too little oxygen in the exhaust and the control module is adding more fuel than normal to sustain the proper air/fuel mixture.

When a vehicle has the fault code P0171 and or P0174, it means that computer can no longer automatically adjust the mixture between air and fuel. Code P0171 applies to Bank 1 and P0174 applies to Bank 2. Code P0174 mainly applies to V6 or V8 engines because 4-cylinder engines generally have only one bank, though there are a few exceptions. Some of the high performance 4-cylinder and straight 6-cylinder engines (such as those in BMW and Lexus) split the cylinders into groups of two or three and call them separate banks.

When the code says that the Fuel System is "too lean," it means that the computer has been adding more and more fuel, which is called Long Term Fuel Trim. Ideally, the Long Term Fuel Trim should be close to 1 to 2 percent. When a code P0171 is set, it means that the Fuel Trim is anywhere from 15 percent to as high as 35 percent compensated. When this happens, the computer knows that there is an improper condition in the control of the Fuel System.

Now for the Causes for these codes:Common Problems That Trigger the P0171 and P0174 Code

PCM software needs to be updated
Vacuum leaks (Intake Manifold Gaskets, vacuum hoses, PCV hoses, etc.)
Mass Airflow Sensor (MAF)
Plugged Fuel Filter or weak Fuel Pump
Plugged or dirty Fuel Injectors

Now many here have gone to the dealerships to get these codes figured out, just to have the vehicle throw the code a week or two later. I found out why!!!!

Now on my scanner, I can check the engine "LIVE DATA" as it is running and it also takes a freeze frame of when the last code was set.
I found that my LONG TERM FUEL TRIM FOR BOTH BANKS (P0171 and P0174) was at 25% or more!!!! This is high enough to cause the computer to throw a code.

Since the issue dealt with BOTH BANKS 1 AND 2, I knew it had to be an issue with my MAF, Vacuum Leak, Fuel related, or possibly gaskets from the intake system.

Here is what I replaced BEFORE I DISCOVERED WHAT SOLVED THE ISSUE:
Below is a photo of everything I replaced minus the 4 - 02 senors and the PCV valve AND MAF SENSOR.
Attachment 429511
Here we see: Numerous Vacuum Lines, Intake Manifold Gaskets, Plenum Gaskets, Valve Cover Gaskets, Spark Plugs, Injector Seals for all 6 Injectors (also cleaned injectors), EGR valve, Air intake control Valve gasket, Air filter, numeruos vacuum line clamps, Crush Washers for fuel rails, and a few other things.

Now this pretty much had me at a loss because I replaced nearly EVERY COMPONENT that could possibly be a culprit to this issue. I am sure that Lexus Techs have tried or done the same thinking "this has to be all it can be".

Well after tearing the engine apart and replacing all the above and servicing and cleaning the injectors and entire intake and throttle body, I was thinking that this will do the trick. I had my fuel pressure tested as well and it was fine (54lbs.). So I knew I didn't have a fuel suply issue from the tank.

Put it back together and turned the key thinking problem solved. AND NO!!!!!!!!!!

WTH MAN! I literally replaced everything. I even inspected the plastic manifld for cracks! Nothing. Still showed High Fuel Trim on the Long Term Side after engine warmed up!

GRRRRRRRRRRRRRRRRRRRRRR!!!!!!!!!!!

What the hell would be next???? I was lost. Went to dealership. Wrote a check for them to diagnose the problem. They did smoke test on system and found no leaks in the vacuum, and reset code and told me system would adjust after a few cycles blah blah blah. They even reset my ECU! ONE WEEK LATER......CODE P0171 and P0174!!!!! AGAIN

At this point I had no clue what to do. Read and read and read about lean codes. Read some more. Inspected all vacuum lines again. Cleaned new maf just in case. Nothing.

Then, FINALLY, AFTER nearly 8 months of fighting with this code KAAAAAAAABBBBBBOOOOOOOOOOMMMMM!!!!!!!! I FOUND THE ISSUE! And I discovered why Lexus techs and mechanics are lost at times. They know it was likely a leak in the vacuum system, but could never clearly identify what was leaking!

I DID!!!!!

While inspecting the engine yet again for potential leaks etc, I was running my hand along the left side of the intake manifold (black plastic) when I placed my hand and fingers and some weight against the AIR INTAKE VALVE on THE LEFT SIDE OF THE ENGINE INTAKE. Now the mechanism and linkage from the vacuum bladder that operates it had my fingers slightly pressing on it and I noticed an engine RPM change and a slight change in the pitch of what sounded like my engine beltpulleys making noise. I release and the engine returned to what is was and had been doing and sounding like. At this point I though to myself that maybe I had simply just moved the linkage enough to move the internal valve inside the intake manifold. So I again placed my fingers near the linage to the valve this time just barely placing enough pressure on the linkage to change the pitch of the noise, yet ensuring I did not MOVE THE VALVE inside. The pitch changed again and engine revs changed slightly. I immediately connected my scanner and drew the live data up. Did it again and pressed on the linkage and BAM THE FUEL TRIMS DROPPED TO NORMAL!!!!!!!!!!!!

BOOOOOOM Maybe this was it!!!!! But how????
This was not a vacuum line, simply linage to the internal air valve. I tested the vacuum line going to the valve bladder (sucked and watched the valve mechaism move thus moving the internal valve and held it and it was air tight. What the hell. How could this be leaking if the bladder holds a vacuum and the linkage works and holds when there is a vacuum????

Well, what the hell, I've already replaced everything else, might as well replace this. TOYOTA PART NUMBER: 17320 0A010 Air Intake Control Valve No 2.Bought it. Tore the intake apart again to get the old one out.
See photo:
Attachment 429512
Attachment 429513
The mechaism to the left my thumb in the photo is what I replaced. It is also picture above.

Now when I removed the old one, I noticed some "play" in the SHAFT THAT RUNS FROM THE CONNECTING LINKAGE THROUGH THE PART THAT MOUNTS TO MANIFLD TO THE VALVE ITSELF. See the photo for a picture of the shaft.
THIS IS WHERE THE LEAK WAS COMING FROM!!!!!!!!!!!!
Attachment 429514
THIS PHOTO is of the linkage WITHOUT PRESSING ON IT.
This photo however....

Shows a slight gap in the shaft area. You CAN CLEARLY FEEL A LOT OF PLAY especially comparative to the new one. There are two rubber seals int he linkage that obviously have gotten old and began to leak due to the play.

But why would the "leak" be intermittant and only throw a code on occasion..... BECAUSE THE VACUUM VALVE HAD TO BE WORKING TO EXPOSE THE PLAY IN THE SHAFT LINKAGE and this usually only actuates at idle at start up or idle while driving. IF YOU WERE NOT IDLEING THE CAR FOR LONG PERIODS, the leak is not apparent and thus the fuel trims are not HIGH ENOUGH FOR LONG ENOUGH to throw a code. At some point they will, but thus the intermittant code throwing!!!! I once reset my code and drove 237 Miles BEFORE THE CODE thre again! It was nearly ALL HIGHWAY MILES....DING DING DING. NO IDLE TIME thus no intense vacuum thus no code throwing.

Now if a Lex Tech hooked up smoke machine, HE WON'T FIND THE LEAK AS THE VACUUM DRIVEN VALVE WILL NOT BE ACTUATED AND THUS THE LEAK NOT APPARENT AT THAT TIME. The rest of the system will check out and you will be back at sqaure one!!!!

PLEASE PLEASE PLEASE Have your Mechanic check this valve mechanism if you are getting these codes. The leak is coming from the SHAFT THAT CONNECTS TO THE VALVE!!!!!!!


After months and months and countless hours, I FINALLY DISCOVERED THE ISSUE!!!!!!!!!!

Email me at htownblue@gmail.com with any questions.

Originally Posted by RedGemini

Odd. Those aren't even close to the codes that most hack mechanics/parts stores want to throw new cats at as a fix. The codes for that are P0420 and P0430. You could try cleaning the MAF sensor with proper spray cleaner. The throttle body may need cleaned as well. Both can be done for under $20 worth of spray cleaners. SeaFoam Spray is great for cleaning the throttle body, though it's tricky without a second person helping. There's a video by SeaFoam showing the proper way to use it. I'll be doing it to mine soon.

Originally Posted by adil1234

I had the same issue with P1071 and P1074, I and my mechanic (a very patient guy), we were trying to figure out, for hours. My 2003,ls430, though it drives smooth, with no issues, check engine light remained. We checked all the oxygen sensors, looked for vacuum leaks, exhaust leaks, etc, and everything seemed to work fine and finally fortunately we thought of opening the air filter box and took the air filter out,it was very very dirty. we got an original air filter and tried, Alhamdlilla,the code vanished.So make sure you check your air filter condition.