Man, diagnosing this P0171 issue has been inbelievable. Here's the latest:

Took it to a mechanic and we talked through what I had done and went from there.

Done before seeing mechanic:

- New MAF
- New upstream O2 sensor
- PCV valve checked and good

When talking to the mechanic:

He ran his handheld scanner and identified that Bank 1 Sensor 1 was reading ZERO volts. In looking at the wiring/connector leading from the harness/connector he seemed to think it was corroded. Once he got in there though he said it looked ok and confirmed that there was continuity all the way to the computer.

Interestingly, he said when he plugged in the scanner and fired up the 4Runner, it read 0.3 volts to Bank 1 Sensor 1 when it was cold, but after about a minute when it began to warm up the reading went back to zero volts.

Additionally, the ECU seems to be operating correctly as nothing else seems to be amiss so I have nearly all but ruled that out as the culprit.

He said his son is at a motor sports technician school in North Carolina and that he was going to ask around and do some research and see if he could figure out the problem.

So WHAT THE HELL IS GOING ON? Any ideas? My only remaining thought is the coolant temp sensor.

Any and all input is appreciated. I'm taking off for Oregon (from Ohio) next Thursday and would love to have this thing remedied, not even really for the increased gas mileage but more so just because it would be nice to know she's running in tip top shape.

Thanks for any and all help.

JT

Is the new O2 sensor OEM?

I am also curious what the fix is. My dad's 4runner has been getting this code. Previous owner replaced the cat, but when we he purchased it it didn't have any codes for over 1k miles when it popped up. Replaced MAF with known good one, replaced pvc, cleaned throttle body, checked for vacuum leaks, and the lastest we replaced the upstream o2 sensor with known good one. Hasn't come on since o2 sensor but it's only been 350 miles since we did that. My dad isn't one to like to throw parts at it to fix it so I've been purchasing new parts for mine as preventative maintenance and giving my old parts to him to see if that is what's wrong. If the up stream o2 doesn't fix it, I don't know what will since the cat has already been replaced.

Did you check for cracked hoses for vacuum leaks?

Yes, it is a DENSO. I cross referenced to make sure I was getting the correct one as well. One cat, 49 state spec.

I have not checked for vacuum leaks. I guess I might as well give that a shot!

New MAF is oem or Denso also? I had a P0171 for a while. Finally fixed it with a new Denso MAF.

Man I feel your pain, a couple years ago I had this p0171 code that would only show when the engine was straining like going up hills. It would go off after a while but show every time I drive. Now I know your problems a little different, but I found a port that should have been capped off ( used for smog on earlier years) well that dam cap came off and finally I just said hell! Capped it off with a silicon cap and have been good ever sense.

I was lucky, it was a .30 piece

Hope you figure it out soon, these headaches are never fun.

Yeah, also DENSO.

Well shoot that's all the input I had. I went through the same thing. Checked everything and replaced front o2 sensor also. MAF finally fixed it. Hope you get it figured out. Popular topic that's for sure!

I had the same code (P0171) and turned out to be the fuel pump went bad.

Car didn't want to start, idled real poorly, and ran fine at speed...

More often than not a P0171 is a vacuum leak. Sorry that you didn't check that one first.

Would the vacuum leak cause there to be a reading of zero volts at Bank1 Sensor 1?

It's possible. If you have a leak and extra oxygen is coming it, it could be maxing the O2 sensor's ability to adjust fuel trims.

Where and how are you reading it? At the sensor, the 12V+ is straight from the EFI fuse and at the sensor, will always be ~12V barring circuit failure. The other side of the sensor goes to ground through the ECM. If the ECM is seeing zero volts output from the sensor and you are reading zero on a scanner, that is likely a wiring or sensor open circuit. At that point, it should set a P0136 or similar code for voltage out of spec, not a lone P0171.

Even at full ambient air oxygen, output voltage from the sensor should max out, or min out I should say, at about .2V. The P0136 cutoff is above that for a continuous reading. A working circuit correctly read should not go to zero.

Bottom line, I'd say it's a wiring issue not a vacuum leak if it's a reported value. If you are using a meter, I'd say you are measuring wrong if the only code is P0171.

Are you a Cali? 2000 Cali's were sold all over the country. That can mean you are reading the wrong PID and that is why you get 0V. Your scanner doesn't know whether you are Cali or not. You have to tell it which PID to use. Check your scanner documentation for A/F sensor readings if you are a Cali.

I hate P0171s.

BINGO!

I've been dealing with this issue for so long I accidentally included P0171 as the code to go along with the zero volt reading when in reality it was P0125. It seems I've been dealing with both of them off and on for the past year.

Long story short: the ground wire from the O2 sensor connector harness (I'm 49 state spec) to the ECU was toast. New wire = no codes, immediate re-calibration of fuel trim on the scanner and voltage reading. MPG are up immediately.

Thank you to all for your suggestions, advice and insight.

Leaving for a 6000 mile road trip tomorrow and couldn't be happier to have this issue solved.