two quick questions. i've done a search and i can't seem to find what i'm looking for.
i think i read somewhere that the P0171 diagnostic code may not be the oxygen sensor. is this the correct diagnosis for the 171?
secondly, is there any thing i need to know about changing the sensor?
thanx
brad

 

PO171 indicates a system too lean code. Your MAF Sensor is probably dirty. Check out this thread for how to do it.

Good luck!
Dr. Z

 

great info.......thanx again

 

will need to have the code cleared, or will the check engine light go off when i clean the MAF?
.............and just to clarify.......i don't need an oxygen sensor? or is the MAF and O2 sensor one in the same?
thanx.
brad

 

You will need to have the code cleared. Go to Autozone, I think they will clear them for free.

You will not need an O2 sensor.

The MAF and O2 sensor are not the same thing. The MAF (Mass Air Flow) sensor is part of your intake system and the O2 sensor is part of your exhaust system.

The MAF tells the engine computer how much air is entering the engine and the O2 sensor tells the engine computer how rich or lean the engine is running.

When the MAF gets dirty, it can't correctly measure the amount of air that the engine is injesting, which causes a too lean condition, since more air is getting in than is being reported.

Hope this helps.
Dr. Z

 

I cleaned the MAF and it didn't seem all that dirty. only the side of the resin ball that's on the back side of the sensor...( in relation to the airflow) no real dirt, corrosion. But i can notice a difference in the pep of the engine!
here's one last question, although it may be premature. i had the code cleared, but the ckeck engine light came on again today. this was the first time i'd ran the engine for any amount of time since the cleaning. any ideas?
i know that maybe an impossible question, but i'll have the code re-read tomorrow to see if i get the same code.
thanx for the prior replies.
brad

 

Here is a good site for determining diagnostic trouble codes

 

that i've got a vacuum leak, causing the engine to run too lean.
how do i track a leak down? any usual culprits? and am i on the right track.
thanx
brad
ps. my code reads p0171 system lean bank one
i haven't done any mods to the engine other than maintaining levels and fluids changes, plugs, and air filters. it's 99 4x4 tacoma with 105k miles.
thanx agian for all the help.... it's amazing how much one learns while trying to track down one problem.
brad

 

Hey,

I'm glad that cleaning the MAF had a positive effect, but I'm sorry you're still having issues with codes. Tracking down a vacuum leak can be a daunting task. The bank one lean means it's the Front O2 Sensor (The one BEFORE the Cat) that is generating the code. I doubt that a bank 2 lean code would be common at all since it's after the Cat and usually the Cat's do their job pretty well.

But, back to your Vacuum leak, you can start by visually inspecting all the Vacuum hoses on and around the plenum. Check them for fit and cracks, etc. Hopefully, it will be visually obvious.

If that doesn't work, the best way to find leaks is to seal the intake system downstream of the MAF and then, using an air compressor, remove the MAF and attach a hose to the place where the MAF mounts and pressurize the intake system. Not a lot of pressure, mind you, but just enough so that you can listen for air escaping. I know that sounds complicated, but it would be the easiest way to track down a leak once you got it set up.

You could also take a length of 1/2" Vacuum hose, around 24" or so and use it like a Stethoscope to find vacuum leaks by listening for them.

I don't know what else to tell you at this point, except to be patient, use a flashlight and scour the engine with your eyes (and ears) to try and find the offending hose if you do indeed have a vacuum leak.

Hope this helps.
Dr. Z

 

Bank one is the odd numbered cylinders and bank two is the even numbered cylinders in your engine, bank one is the left o2 sensor and bank 2 is the right o2 sensor, if you're looking for a vac. leak, spray some carb cleaner around the engine one section at a time, that will help you to at least narrow down the area where the leak is, because the idle will dip when the carb cleaner is sucked in.... good luck, but my best guess is you need injectors...

 

with that info, i think i'm gonna change the front 02 sensor and see what that gets me. i wanted to bounce the vacuum leak off of you to see what you thought. i can neither hear or tell if there are any leaks over any part of the engine, plus my idle speed actually went down a little after cleaning the MAF sensor. i'm thinking i'd idle fast if there were a leak.
you guys are great to help!! i'll let you know this weekend if the 02 sensor worked.
thanx again
brad

 

Not on these trucks it isn't. The O2 Sensors are AFTER the X-Over Pipe. One is right before the Catalytic (Bank 1) and the other is right after the Cat (Bank 2). So, BOTH O2 Sensors measure BOTH Cylinder Banks.

No offense, but your answer applies to a different type of setup than these trucks are running.

Peace.
Dr. Z

 

Okay, Brad. Let me know how it goes.

Peace.
Dr. Z

 

dr. z., i did not know that, but as i understand it, bank one would still mean one side of the engine, and bank 2 the other side... which in my ford training leads me to believe he needs injectors, and at 105k miles... it isnt a surprise, at least not to me.... good luck with the o2 sensor, hope it fixes it, as it is the least expensive option...

 

That's alright. On OBDII vehicles, the O2 Sensors are set up that way and Bank 1 means the Sensor in front of the Cat. It has to do with the trouble codes generated and not with the Left/Right orientation of the engine.

I don't think he has bad injectors. They MIGHT be clogged. But he could just have them ultrasonically cleaned at NAPA for far less money than a new set.

I think that if the new O2 sensor fails to fix what's wrong, then that should be the next step in the process.

Peace.
Dr. Z

 

obd2 cars? all of them? cant be right, rangers arent that way, neither are explorers or f150s... maybe its a ford thing?

 

Are you talking about vehicles manufactured AFTER 1996? Because that's what I'm talking about.

If those vehicles have an O2 Sensor on each cylnder bank, then they must have one after the cat as well.

It might just be a Ford thing or only the V8 engines.

Dr. Z

 

I guess I was thinking of engines with only 2 O2 Sensors. Which is the vast majority of OBDII engines. I forgot that my brother-in-laws Maxima has at least 3 O2 Sensors. My bad.

However, on our trucks the Bank 1 is the upstream O2 and Bank 2 is the downstream.

Later!
Dr. Z

 

i'm listening and learning

 

Dr. Z., thanks for clearing that up, now i know... good luck bama...