This is what I got yesterday evening.
BTW I did install K&N filter this summer (if that’s what you meant by K&B).

And this is what I get when the engine is idling

Continued...

^^^
Thank you for this info.
In all honesty, your trims are almost perfect at idle.
MAF readings are also good.
O2 sensor fluctuations are sound.
Only thing im not seeing here iare the AF voltages.
I see a O2S1 and O2S5 both at 1.0v.
Those could be AF FT voltage. (1.0v is actually good)
Im looking for AFS voltage.
Value of around 3.30v at idle at each bank.
See if you have this data.

One more thing, when you get these LEAN codes, can your scanner see the freeze frame?
That would be helpful as well.

The MAF is reading roughly double correct Air Flow Rate the voltage swings are too exaggerated. Your Sensor codes are set and you’ve cleaned these codes before with CRC on the MAF. You’ve got an oiled Intake Air Filter (booo...) coating the entire Air Intake System; Air Box, MAF, Air Tubes and the Throttle Body coated in this stuff. These components are designed to be operated bone dry and clean. Clean them and remove the Throttle Body and clean it in hand. Not hard. The back side of the butterfly gets absolutely filthy.

Your O2 Sensors aren’t broken and they’re trying to keep up, but they’re doomed to fail or go “suspect” soon because of fake readings.

While you’re doing some low cost maintenance, change your PCV valve. Since this component is on most every list of items to check in P0171 code readings and is one of the items that often gets neglected, spending $10 on a new one isn’t throwing parts. IMO, everything is pointing towards the common point of the Air/Fuel Intake System.

When you’ve got it cleaned up and back to a dry air filter, clear your codes and take a ride. Your Fuel Trim levels will relearn themselves as you drive and hopefully your lights will stay off.

Nad1370,

My bad. Long day, late hours and my old ECM was suffering from too much TMI. I did respond last night, but never hit the “Submit” button and when I hit it this morning, my internet posting session had timed out, so here I am for a bit.

Basically, the MAF g/sec rate goes from OK at Idle to double the engineered rate at 2500 rpms. 2 - 7 g/sec is good at Idle (600 rpms), but at 2500 rpms it should be between 15 - 25 g/sec. See Fortitude’s Post #22.

The Short Term Trim Rates for both banks by the O2s indicates more gas is being injected to “fix” the lean condition. In positive corrections, fuel is added to adjust for a lean condition, while negative corrections respond to a rich react very quickly to O2 sensor input.

The Long Term Trim Rates are roughly twice the normal and high Positive Trims/positive fuel corrections can be traced to MAF and O2 sensor faults, vacuum leaks from intake gaskets/hoses, unmetered air (intake snorkel leak), clogged or dirty fuel injectors, fuel delivery issues and even a clogged catalytic converter.

In positive corrections, fuel is added to adjust for a lean condition, while negative corrections respond to a rich condition. STFT corrections represent and react very quickly to O2 sensor input. If you were to create a large vacuum leak and disconnect the PCV hose, the computer would immediately add positive fuel trim to balance the mixture. Short Term Fuel Trim is not stored in Keep Alive Memory (KAM) in the ECM after shut down and automatically resets to 0 for the next start/run cycle.

I agree with your question as to why AF Sensors aren’t considered more than they are in these articles. And given this car has 178K on it, it’s a valid concern. Maybe because the Air Intake System doesn’t get enough “user maintenance” and they tend to go bad first(?). Take the car in question here... It has a K&N oiled Air Filter and they’re getting a baaad name for funking up not only MAFs everywhere, but now it’s from the Air Box all the way to the injectors with their goo. The car here has no mention of maintenance history other than cleaning the MAF to clear out a P0171 code a time or two and a P0174 code following along soon(?).

Your recommendation in Post #5 is where we’re at... Time to get our hands dirty and this is cheap and easy stuff.

1. Get rid of K&N Air Filter and replace with Genuine OEM.
2. Clean the Air Box and Air tube with Ammonia-free Windex.
3. Remove MAF and clean with CRC.
4. Clean inside the MAF mounting area.
5. Remove Throttle Body and clean with Carb Cleaner and thoroughly rinse after with CRC.
6. Spend $10 and change out often neglected PCV valve with Genuine OEM.
7. Spend $15 more and change out neglected Fuel Filler Cap with Genuine OEM, be sure to silicon spray the rubber sealer gasket first.

Do these things and get back with us, please.

Thanks to mirekti for the pictures and thanks to Fortitude for the read out.

Cheers

Guys, first of all all thanks goes to you. I really appreciate all the effort.

I am not sure what to say right now. It’s been three days since I haven’t received any codes, and they’ve been coming within hours after clearing them in the last two weeks.

So far I have only replaced the tank cap. MAF sensor arrived few days bac, but I still havent replaced it as I was waiting for the OEM air filter. PCV is supposed to be delivered today.

I will certainly replace PCV, but not sure about the MAF and K&N filter anymore. It’s been rather cold last two days here. This makes me think one of my hoses might leak and now, because it is cold, they shrinked and the codes don’t show up (just the flow of my thoughts, not that I am certain this is happening).

Oh, there’s one more thing. My wife had been driving the car last three weeks too, and it happened that she had ended up at a gas station each time the car needed gas. For some reason she was not pouring Premium gas, but Regular and the one in between. Could that have been the cause for lean codes? I only found this out two days ago by chance, and aprat from the colder weather the car runs on Premium since Friday, and no error codes.

I will keep you posted.

No... Thank YOU for coming back with a bit of progress and telling us what’s next. And believe me when I say busting knuckles gets tough when it’s cold. The MAF is an easy one to replace and it’s certainly your choice on the K&N Air Intake Filter vs Lexus engineering, and there’s a bunch of web info to help you decide.

Glad you replaced the Gas Cap and hope it was a Genuine OEM part? Spray some silicon spray on the cap’s rubber washers.

I run premium gas gas because my car runs better, has more pep when I want it and premium gas generally contains more cleaning additives than regular. Here’s a tip: Get a 6oz bottle of Gumout® Regane® High Mileage Fuel System Cleaner and add it to your gas every 3 - 5,000 miles. Good stuff.

Keep us posted.