Another P0171 Thread...2000 2.7L PreRunner

OK, so I'm another afflicted owner of a 2000 PreRunner 2.7 automatic with 160K miles and a P0171 code (lean on Bank 1). Cleaned the MAF. No luck. Replaced the MAF, spark plugs, fuel filter and air filter. No luck. Code still resets almost immediately after a few miles of driving. Checked for vacuum leaks. None that I can tell.

What next? PCV? Are oxygen sensors ever the culprit? Are oxygen sensors the same as air/fuel ratio sensors on this truck? Could the sensors be telling the ECU that the exhaust is lean when it isn't? Common?

 

http://www.customtacos.com/tech.old...06toyrm/06toypdf/06rmsrc/rm2006ta/0050027.pdf

 

Thank you. I was afraid of that.

Where is the A/F sensor located? Anyone know?

 

More information. So I look at the data stream that my Launch CRecorder recorded and I see that the B1S1 sensor (also called the air fuel ratio sensor in a California equipped vehicle) is stuck at .66v. Never deviates from that position. I'm assuming that this is not right since it's basically an oxygen sensor and values should be fluctuation. However, I'm wondering if it's stuck there because whatever gases its sensing are at a level that puts the sensor at the bottom of its value range.

Anyone traveled this road before? The chart someone posted earlier seems to indicate that a defective AFS could cause P0171 if it's artificially detecting a lean condition that doesn't really exist.

Help? Advice?

 

Sorry can't help with that particular CEL or your fixed voltage. In terms location, check out this tutorial: http://www.4x4wire.com/toyota/4Runner/tech/O2_Sensor/

Do you have a K&N? A quick google search for "P0171 Tacoma" brings up a ton of threads talking about K&N issues. It also brought up MAF issues which seems like you tried replacing. Have you ever replaced the A/F sensor? How many miles?

 

Thanks. I found it. It looks exactly like an oxygen sensor and is just downstream from the exhaust manifold and pre-cat.

Talked to a dealer shop foreman yesterday who said it is not right for an AFS to have fixed voltage like mine does (pretty much already knew this). In order to know for sure, though, he recommended running the engine while at operating temp and squirting a blast of carb spray or Brakleen into the air induction system to see if the AFS voltage reacts. If it doesn't, it's definitely not working correctly and is likely the cause of my P0171. Let's hope. That's an easy fix.

 

Looks like the new air fuel ratio sensor cured my P0171. The check engine light is off and the new long and short term fuel trims are right where they should be. But it was not without a bit of hassle replacing the sensor.

One of the sensor nuts was frozen to the stud and didn't want to come out. A bit of muscle had the stud backing out of the pipe, but only about 1/2 way before it broke off flush with the pipe. Had to remove the down pipe it and have it drilled out at a machine shop. As I was trying to remove the down pipe the nut holding the pipe to the manifold also froze to the stud and semi trashed the threads in the manifold. I thought I might have to remove the manifold and re-tap it but luckily I was able to clean the threads up with a tap coming in from the engine side of the manifold and installed a new stud.

FYI, Toyota doesn't sell the air fuel ratio sensor studs separately from the pipe (and the down pipe is a cool $1300!!!). They do sell the manifold studs that connect to the down pipe, though, so I was able to install the correct part for a clean reinstallation.

Thanks to the Launch CRecorder, too, as it was an easy to use, inexpensive ($90) tool that let me diagnose instead of parts-change in a trial and error fiasco.

 

Don't forget to spray with PB blaster a day before to get those nuts off easily. In case you run into more problems, I just saw this thread that also cured a P0171 CEL: http://www.tacomaworld.com/forum/1st-gen-tacomas/330233-p0171-solved-finally.html