So, I've read everything here at Club Lexus on system too lean, O2 sensors and cleaning the MAF. I have now replaced all 3 sensors as well as the MAF...all with OEM parts. The car seems to be running fine, but I just got another P0171. I cleared the codes and reset the CEL but it came back on 2 days later.
So here's my question...I noticed when I was checking the air filter, that the entire air intake section seems unsecure. If I put my hand on the air filter casing and shake it a little, the whole area seems to move more than it should (at least in my amateur opinion). Could that be causing an issue?...maybe an air/vacuum issue? Or is it normal for the air intake area to be wobbly?
I think I know what everyone is going to say...so, is there an easy fix to better stabalize those plastic pieces? Could it just be dislodged and maybe I just need to pop something back into place?

 

I have a 2001,and my air-box is loose like that too.I don`t think it`s a real concern.Just make sure that little hose behind the air-box,is in place,and secure.

 

Is it a P0171 by itself, or does it also have a P0171 PD (or pending)?

 

I believe it's just a P0171 by itself. I took it to AutoZone and watched as the tech did it...I don't remember there being anything on the screen other than P0171.

 

What were the driving circumstances (mph, where, what time of day, temp) when this light has come on?

 

I'm in Wisconsin, so it has been a cold winter. On average, the cold engine starts have been around 20 degrees in the mornings and evenings commuting to and from work. I live out in the sticks...unfortunately, it rarely gets to cruise at 55+ mph...usually I go 30-40 mph with lots of stop & go's. Avg mph on the computer says 26.
The last time it came on was in the early evening after it had been sitting for 2 hrs in a parking lot at 30 degrees outside temp...driving on a suburb surface street at 35 mph.

 

The reason I asked that is because the temperature can play havoc and cause this particular sensor error to occur. I get it also down here, where we have been suffering 50 degree weather, and the altitude makes it feel even cooler. If I step on the accelerator and then quickly pull off and let it coast, I will throw this error (that is something unique to me for my air modifications). I can get this code at higher speeds, but most often it occurs just when I am pulling out down the street.

I'd pull the battery cable and reset the system and let the warmer weather come.

 

Now, you mentioned that you replaced all three sensors... Did you replace the O2 sensor and the the 2 Air/Fuel sensors? or are you under the impression that you replaced 3 O2 sensors?

 

I took it in 4 times for new sensors, 2 on the engine, 1 by the CAT and the last one was the MAF.

 

Any Lexus/Toyota or for that matter any mechanic with an advanced OBD2 sensor can read the reading off of those 2 A/F sensors to see if they are in the right specs. Sometimes, injector, blockage, even minor with new A/F sensors can cause a P0171 (since slightly more air than fuel).

 

4 times? Provided that you took into a knowledgeable source, they should have replaced both a/f sensors with the right sensors and then the O2 sensor. My friend with his ES300 had a local shop replace sensors due to the same code. Nonetheless, they put A/F sensors where the O2 sensor goes and he received the same code again 2 days later. Just too similar of a situation.

Again, how is your engine running? 26 MPG with the described driving style is a little too good. Should be at around 18-19 MPG. Maybe your lean situation is helping with fuel consumption, but in the long run it will hurt the engine.

 

OK...new update. Two weeks ago, the CEL came on for the first time after replacing the sensors. At the time, the tank was less than half full. I filled it up and within 24 hrs the CEL turned off by itself. I didn't think anything of it...just assumed it was a temporary thing and the ECU made an adjustment. This week, when the CEL came back on, the gas tank again was less than half full...but again I didn't think that would be the issue...however, this afternoon I filled up again and within the first 10 miles the CEL turned off again on its own. So now, I have a noticible pattern. When I had the emmissions checked 30 days ago, they checked the gas cap and said it was fine...any ideas?
I'm getting closer to the culprit!

 

Weird. Not sure if the fuel pump is not working right and not sending enough fuel after it drops to a certain level...

 

Run some Redline Fuel System Cleaner on a 1/2 tank, your injectors and pump may have some crud on it.

I can tell you that when I do get the P0171 error on my RX, it always happens below 1/2 tank, generally 1/4 tank, but the error goes away for a few weeks after I have run something like either Techron or the more potent Redline cleaner (AutoZone should have it).