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I have an 06 Lexus with 205k miles that is tripping both a p0171 and p0174 diagnostic codes. They infer a lean gas air mixture and the short term and long term trim values reflect an attempt to make corrections. They also decrease with elevated rpm which can be an indication of a vacuum leak.

The bank1 and bank2 o2sensor readings are both 0.7... volt which infer a rich mixture.

I assume the o2sensor readings are 'upstream' os sensors.

AIUI a downstream(exhaust side of catalytic converter) o2 sensor is critical here and is the major control input to the emission control unit(or whatever it is properly called) that adjusts the trims.

So my question(s) to any automotive guru out there are:

How can the trims be trying to resolve a lean condition while the o2 sensors indicate a rich condition?

What might my scan/diagnostic tool be calling the post cat convertor os sensor. Knowing what that output is might solve the first question.

I really need to monitor the down stream o2 sensor output. WTH is it called??? AIUI 0.450 v is the indication of a sc........... mixture.


HELP! please

Have you tried the Chilton's manual? I know that they do not print their manuals much anymore but they are online. Some libraries have a way for their members to access the site. I believe it is through Gale. It just may be a little confusing on how to get to it.

JPG,

Have you tried the Shopsmith forum, err, uh, wait a minute, nope, sorry the Lexus forum? I did a Google search on those error codes (p0171 and p0174) and found this rather interesting post on the Club Lexus forum, a site that I have found useful myself a couple of times in the past. Not saying his solution will fix your problem, but he does a great job explaining the troubleshooting he did for those error codes on his Lexus. Beyond that I won't be much help for you. Been a few decades since I chased an O2 sensor problem in a Ford Escort, so memory banks are hazy. Good luck with it though.

https://www.clublexus.com/forums/rx-2nd ... codes.html

WHEW!

Very interesting reading. Will examine acv closely as well as calculated load. Both new paths.

I was hoping to find out what the ecu input post cat conv was called. So that remains a question - maybe the new path will eliminate the need to know.

Thank You VERY much for that link.

JPG wrote: ↑Sat Jan 11, 2025 12:49 pm Thank You VERY much for that link.

JPG,

I got lucky that I found that one on a simple Google search. Too often, I have to hunt for what I am looking for, especially now that AI is being used as part of the internet search algorithms. Fingers crossed that this new direction isn't a wild goose chase for you. Sometimes auto troubleshooting, particularly on more modern vehicles is an endless decision tree until one day you find the solution. Hope you find it quickly.

Trying to remember my Chevy's glitch from the 2024 summer. Whatever it was, once I got some Shell V-Power 'detergent' gas and really wrung out the engine, my fault went away. During the fault, I inferred the exhaust was deemed "rich" and the control system kept reducing fuel injector duty cycle. Likely one of the (dirty) injectors was dumping extra fuel in the cylinder, even while driven at reduced duty cycle.

I'm just glad I got away with no new parts!

Thanks to all who responded.

Done some more 'research' last night and gained some understanding. Finally got to useful information(relevant, accurate, understandable postings)

AIUI my 06 Lexus has both 02 sensors and air/fuel sensors.

All the sensor input values are in typical range(assuming conclusion below is correct) so no sensor is malfunctioning.

Trim values are indicative of a lean situation derived from mass air flow sensor and air/fuel sensors at the exhaust manifold. The ecu is responding by adjusting the internal fuel trim values which alter the injector timing(duration).

The air/fuel sensors are determining the correct mixture ratio is near optimal.(that "s" word)

However the o2 sensor(s) post catalytic converter) are detecting low o2 presence. Hence a 'rich' condition(that clears up my confusion re o2 sensor indicating rich).

But BUT but the trim values are near/at maximum which leads to the final path to take to find the cause - find the vacuum leak. It is the repetitive trim values that triggers the diagnostic trouble codes(check engine/vsc/track dash lights) which indicate P0171 and/or P0174 .

Now a vacuum leak was the initial suspect, but those sensors malfunctioning could have affected the trim values as well.(garbage in / garbage out)

Suspecting a malfunctioning sensor with no proof(only assumptions re that IS the cause) can/will/has lead many down the wrong rabbit hole with expensive sensors being replaced with no change in the symptoms.

THAT is what I wanted to avoid, but did not know what/where the sensors should have been indicating.

Finally last night I was exposed to the air/fuel sensor details and it all came together. Testing air/fuel senor with a diagnostic tool requires a trick. The trick is mentioned elsewhere but to serve different purpose(s). Short open throttle followed by rapid throttle closing. That causes the normal steady air/fuel sensors to respond with a predictable variation. There are other 'tricks' to determine sensors proper reactions).

Drove today a couple hundred miles and the sulfurous exhaust stench confirms rich air/fuel ratio.

So after returning home an exhaustive (pun intended)vacuum leak search shall commence(weather permitting).

Will let all know the results(or why I need to go back and start over).

JPG wrote: ↑Sun Jan 12, 2025 10:49 pm

Drove today a couple hundred miles and the sulfurous exhaust stench confirms rich air/fuel ratio.

So after returning home an exhaustive (pun intended)vacuum leak search shall commence(weather permitting).

Will let all know the results(or why I need to go back and start over).

After you Phrased it like you did it brought back memories of
I had a vehicle that gave me the run around on codes like this similar situation you are having with the O2 sensors.

I drove and did cycle time stuff Etc. Etc. Etc.

I found the issue finally.

The passage under the EGR valve was blocked.

I don't know if this is an issue with your particular engine but just food for thought.

That would only cost a gasket or 2 to find out.

Well go bring this to a happy conclusion:

Yes I did have a vacuum leak in the bellows looking air duct(two of them) down in the valley of the bellows. Replacing them made the trim fail codes(P0171,P0174) disappear and the long term trim to a reasonable value(about 15%). Reasonable I assume for a 200k+ motor.

AS I now understand it the air/fuel sensors(a later version of an O2 sensor) were doing their job which correctly resulted in a Stoichiometric(sp?) condition, but only by increasing the injector timing by way of the long term fuel ecu output. The computer in charge determined that the long term trim values were excessive and set the P0171, P0174 diagnostic test codes.

As for the post catalytic converter O2 sensor, it was verifying the Stoichi.... mixture by indicating a rich condition(all the available oxygen consumed by the catalytic converter).

So all is now happy and it runs SOOO much better/smoother than a month ago. I think the fuel consumption has decreased as well.

Lesson learned here by me is that stated by Benjamin Franklin that "one can never be sure of what one reads on the internet". A lot of well intentioned but unguided info as well as things today are not as they were when a post was initiated.

Looking back I am amused by the "advice"/"fixes" offered that did not directly address the problem, but merely caused the ecu to undergo a history depletion.

Lastly I am impressed by the development of the devices that make all this (pollution minimization) possible.




Now to determine why the air bag light has become a common occurrence(not always). Gotta locate where the thingie that determines whether the seat occupant is an adult or a child that is not behaving correctly. Gotta be under or near the passenger seat. In the mean time the air bag will not deploy on that side.


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