Introduced P0171 by Changing Upstream Air/Fuel Ratio Sensor

[aawolf]

Hey Guys-

I've been working through the suggested tune ups on my recently purchased 2000 V6 Auto 4Runner. I'm seeing city gas mileage in the 10-13 MPG range.

Today I changed the upstream A/F sensor (I've got a California spec 4Runner).

After switching it out, truck started normally and no CEL came up until I went for about 1 mile on my test drive. Then it threw P0171. I cleared it using Automatic, and test drove again and it came back after about the same amount of time.

Question is: can replacing the upstream O2 sensor reveal legitimate problems that were previously going un-reported by a faulty one? Is it possible that I installed this improperly? If the O2 was not fully plugged into its harness is this a code it might throw?

Thanks!

 

[Paperjam]

P0171 is set when the O2 sensor detects a lean condition on bank one. This has nothing to do with the sensor itself likely unless you got a defective part. There has to be something causing a lean condition on that side of the engine via a vacuum leak or there is air entering the exhaust system pre 02 sensor. Do California spec trucks have air pumps on them? Why did you replace the sensor originally?

Also, if the sensor was shorted previously or was not producing enough voltage it could "hide" another issue as far as the self diagnostics go. Have you noticed any other running issues? High idle, lacking power etc.

 

[TheDurk]

If the new sensor did not seal tightly, exhaust gas leakage at the sensor could set a P0171. (see attached). Did you clean the mating surface to bare metal, install the gasket correctly, and torque the nuts to spec?

Also, did you use a Toyota or OEM Denso sensor? Weird stuff happens with other brands.

 

[aawolf]

I replaced the sensor to address poor gas mileage. The truck has 162k and the sensor was very likely never changed.

if the sensor was shorted previously or was not producing enough voltage it could "hide" another issue as far as the self diagnostics go. Have you noticed any other running issues? High idle, lacking power etc.

This is what I was wondering, mostly. This would explain why I had no CEL with my old sensor and immediately pulled a P0171 with the new one. The P0171 condition was there already, just my A/F sensor wasn't reporting it.

I noticed also that the exhaust seems rich with gasoline smell now. It's also a very unscientific measurement at this point, but my gas mileage seems abysmal. Could this be that the new sensor is now reporting back false data and the rest of the system is responding? Perhaps the new sensor is defective?

 

[aawolf]

If the new sensor did not seal tightly, exhaust gas leakage at the sensor could set a P0171. (see attached). Did you clean the mating surface to bare metal, install the gasket correctly, and torque the nuts to spec?

You know, me screwing up the gasket installation seems like a very likely possibility Durk. It also fits with what I thought at the time was gasoline smell from the tail pipe. It might have actually been gasoline smell from BEFORE the tail pipe.

I placed the gasket on there blind, and only when l got out from under the truck did I realize the old gasket was not identical on both sides. Whoops.. Will check this first thing.

Also, did you use a Toyota or OEM Denso sensor? Weird stuff happens with other brands.

Yup, OEM.

Thanks, again... Me screwing up the install seems a lot more likely than a defective sensor.

 

[Paperjam]

Ok, so when the PCM detects a lean condition it will adjust the fuel trims to compensate for this condition by lengthening the pulse width of the injectors. More on time means more fuel to try and hit that stoic sweet spot (14.7:1). This explains the gas smell, the truck thinks it's lean when it's not. I would check (as mentioned already) the sealing surface of the O2 sensor, I would also check the gaskets at the downpipe and exhaust manifold for leaks. As excessive oxygen entering the exhaust system before the primary sensor will fool it into thinking its lean. It's odd that it's on only the one bank, if it were a running issue it would be likely it will set lean codes for both banks not just the one, also the fact that it sets the code immidiately on one bank is suspect of an air leak.


If this truck has an air pump, it may direct fresh air to the exhaust manifold to burn off excess hydrocarbons during startup when the engine is in open loop, it could be leaking air into the manifold when it's not supposed to.

I have zero experience with cal-spec 4runners, so it may or may not have the air pump.

 

[TheDurk]

Ok, so when the PCM detects a lean condition it will adjust the fuel trims to compensate for this condition by lengthening the pulse width of the injectors. More on time means more fuel to try and hit that stoic sweet spot (14.7:1). This explains the gas smell, the truck thinks it's lean when it's not. I would check (as mentioned already) the sealing surface of the O2 sensor, I would also check the gaskets at the downpipe and exhaust manifold for leaks. As excessive oxygen entering the exhaust system before the primary sensor will fool it into thinking its lean. It's odd that it's on only the one bank, if it were a running issue it would be likely it will set lean codes for both banks not just the one, also the fact that it sets the code immidiately on one bank is suspect of an air leak.


If this truck has an air pump, it may direct fresh air to the exhaust manifold to burn off excess hydrocarbons during startup when the engine is in open loop, it could be leaking air into the manifold when it's not supposed to.

I have zero experience with cal-spec 4runners, so it may or may not have the air pump.

Only one exhaust bank on 3G 4Runners. 4G has two. No air pump, either.

 

[Paperjam]

ok. I'd still have to say by the fact that it's running pig rich and throwing lean codes i would suspect some kind of exhaust leak.

 

[aawolf]

Thanks paperjam.

With this post I'm really only trying to analyze why the P0171 would appear after switching to the new (identical OEM) A/F sensor.

From your responses and TheDurks I've gathered:

1) the new sensor could be defective
2) the old sensor could have been disabled in such a way that it was hiding pre-existing problems
3) my installation of the new sensor could have been defective

I'll be looking into 3) in an hour or two and I'll report back. It seems like a loose connection at the A/F sensor's gasket would explain everything: the sensor itself is getting ambient air directly from the lose gasket and is causing the sensor to detect "lean" and the rest of the truck to richen up the fuel lines inappropriately (hence the gasoline smelling exhaust).

I just moved the truck without letting it warm up / going closed-loop and I didn't smell any gas near the sensor, or from he exhaust. I think this clue could still be consistent with cause 3).

 

[Paperjam]

Just make sure your exhaust flange surface is nice and clean and flat and no debris is in there. If you have to there is a trick to getting gaskets to seal that I use in my shop all the time (for exhaust gaskets only). Apply a thin coat of anti seize to either side of the gasket and re install after cleaning, DO NOT use silicone. Don't gob it on but just lightly coat it, i e managed to re use gaskets that were broken in half and still get it to seal.

To find pesky exhaust leaks I use a smoke machine normally, you might be able to DIY one, you basically blow smoke up the tailpipe (engine off) and look for any smoke leaks.

Good luck

 

[aawolf]

TheDurk's initial hunch was correct. I'm messed up by not torquing down the A/F Ratio sensor bolts enough. This time I bought a 12mm ratcheting wrench and used that to much better effect. No P0171 light has returned in ~40 miles of driving (whereas it was instant once car was warm before).

I'm still seeing not great gas mileage, but it seems improved. I also changed out the Fuel Filter, cleaned the MAF, and replaced air filter back to OEM. After resetting the EFI map maybe I just need to drive a couple of tanks for it to improve?