Code P0171- B1 Too Lean

[Domenico Nelli]

Hi,

I have had the check engine light (CEL) illuminate in my 2000 RX300 with 100K miles. I live at sea level. I ran it up to Advance Auto to scan it and the one code that set was P0171. I read on this forum, that it is a mass air flow (MAF) meter sensor. So, I went home after clearing the code and cleaned the MAF with MAF cleaner. I had to run out shortly after reassembling the MAF and within a mile the CEL came back on!! I drove the RX back up to Advance Auto yesterday evening to read the new code. I had another P0171 (B1 too Lean) set with a second P0171 "Pending". The customer service person said that this particular scanner (ACUTRON)says either lean or weak. So it was explained to me that "lean" meant that I was receiving too much gas and weak would mean too much air?!? (I thought lean meant too much air and rich meant too much fuel.) I went home again, disassembled and re-cleaned it a little more thoroughly. While I had the MAF out, I did a resistance check between E2 and THA terminal's as per the SFI_SF_file someone was kind enough to post. It says, on page SF-31, that the resistance at 68 degrees F should fall between 2.21-2.69 kOhm. I measured 1.188 kOhm's @ ~72 degrees. It seems to be out of spec. The tech manual also has a test to inspect the MAF operation. The voltage did change while I blew air into it while plugged in, with ignition on but the difference was in millivolts. I have put 35 miles on it since yesterday evening and the CEL is still out... Also, I spoke to a couple tech friends yesterday and each of them say the code is related to either the A/F and or the O2 sensors. This forum says that, they are common misdiagnosis. Is this true? How many A/F sensors do I have, 2? How many O2 sensors are on this car, 1? What exactly does a pending code mean? I mean, will the same code set a second time? Which cyl head is bank 1 vice bank 2? Should the car be scanned with a more sophisticated scanner in real time? Does anyone have any other suggestions as to what it may be? The vehicle has been reliable, though I had a main seal replace at around 62K under a good will warranty. And a stuck throttle one 12 degree morning I decided not to warm it up (this happened one time only, a few years ago). I suppose I will ride the warmed up RX until the light comes back on.

Thanks for any help you could lend.

SFI__SF__file.pdf

[MTOM01]

I had the same problem, I have replaced oxygen sensors but the problem never went away. I have also tried cleaning the MAF sensor, it worked for a while but came back after a week or so.

I replaced it with our Toyota Corolla's MAF sensor which was totalled at 5K miles. This was about a year and half ago.

I have no issues since then.

Seems that all Toyota MAF sensors are interchange.

Take the RX MAF sensor to a Toyota dealer so u r sure the part matches.

Hope this helps.

[Domenico Nelli]

Thanks for the reply. What year and engine type was your corolla for reference? Also, what year is your RX? Lexus charges you top dollar, as you know, for parts and they don't even re package in Lexus packaging!! I mean they could at least make you feel like your getting a LEXUS part...for the price. If the light comes back on.. I may price out the corolla MAF...

Thanks

[davidditt]

I had the same 0171 and cleaned then changed the MAF. Still had the exact problem. Also had 1130,1135, 1150, and 1155. It was running horribly and I decided to take it to the dealer. I was afraid I wouldn't make it because it was running so poorly so I thought maybe something was causing the plugs to foul, so I took one out to clean it. The gap seem way to big so I purchase the three front ones and replaced them because it was easy. The 0171 error is gone and it runs great. 1130,1150, 1135 and 1155 are still there but runs okay. I was just on the forum to see if I was man enought to change the back three plugs because I'm assuming they need to be changed. On that note will it damage anything to run different plugs in different degrees of wear?:

[Domenico Nelli]

On that note will it damage anything to run different plugs in different degrees of wear?:

I had a Lexus tech buddy tell me that the A/F sensors and such should be done at the same time...as for the plugs..it is less than ideal to be running three good plugs and three beat plugs...or potentially beat plugs...you may want to consider changing the rear three....good luck with that.

[iolmaster]

About a month ago I went through the same process. I tried leaning the MAF with no luck. I have 130,000 miles on mine and I decided to just buy a new MAF. I got a reconditioned one at Advance and installed it and cleared the code and the light has not come back on. The new MAF did the trick. I know it is popular to clean these things and that should be the first try. However, they do go bad. The reconditioned MAF is a Denso and it cost $79.00 at Advance with a $50.00 core charge.

[Domenico Nelli]

About a month ago I went through the same process. I tried leaning the MAF with no luck. I have 130,000 miles on mine and I decided to just buy a new MAF. I got a reconditioned one at Advance and installed it and cleared the code and the light has not come back on. The new MAF did the trick. I know it is popular to clean these things and that should be the first try. However, they do go bad. The reconditioned MAF is a Denso and it cost $79.00 at Advance with a $50.00 core charge.

Thanks. The CEL is still off and it has been about 125 miles since I cleared the code... I thought I saw a fuzzy, like from the air filter, deep within the MAF sensor the first time i cleaned it. So this last cleaning I went in there and gingerly wiped the two wires with a de fuzzed cotton swab. Then I blasted it with MAF cleaner. Hasn't been on since. Hopefully, it'll stay that way...lol