By on December 2, 2009

Beats me! (

TTAC Commentator xyzzy writes:

One of the O2 sensors on my 1998 Lexus LS400 recently failed, throwing a P0161 OBDII code (at 175K miles). I’ve cleared the code and verified it comes back, so I don’t think it’s a transient failure. I haven’t yet replaced it, but I have noticed that my gas mileage, as reported on the trip computer, has improved significantly since the failure. Before it was 23-24 mpg, now I’m looking at 26. I know that one of the functions of the O2 sensor is to help regulate mixture, so I’m guessing the mixture is now leaner without one of the O2 sensors. I’ve noticed no discernable change in performance of the car. Assuming I can clear the error code before taking it in for OBDII emissions inspection, is there a downside to just leaving it and running aggressively lean (if that’s in fact what’s happening)?

Sajeev answers:

If your O2 sensors have 175k on their tickers, you’re asking the wrong questions. But the answer to all questions is the same: replace the O2 sensors. That’s all four of them, partner.

Running old O2 sensors is like running an out of tune carburetor. Some manufacturers recommend changing them after 50k for peak efficiency, but I’d recommend every 100-150k for any application. (i.e. When your ride goes in the shop or you are motivated to spend some quality time underneath it.)

Even if the sensors are “good,” you can’t clear an OBD-II error code and pass an inspection: do so and the service tech gets a message saying the vehicle isn’t ready to test. So they’ll take your money for the privilege, then promptly fail your car…and ask you to come back after a week of driving. And, again, the code shall magically resurface.

Or conversely, it behooves you to do the right thing. Bite the bullet, and do a tune up (O2 sensors at the bare minimum) on this car.

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21 Comments on “Piston Slap: The Oxygen Network...”

  • avatar

    If the fuel mileage has improved that much and the reason is a leaner mixture, I’d be worried about what that leaner mixture is doing to the valves and piston crowns. Yes, replace the sensors.

  • avatar

    Agree with all of the above (change out the O2’s).
    The mileage improvement would have to be confirmed with pen and paper though.
    I highly doubt the mileage is better with a malfunctioning O2.
    Perhaps the computer is not getting proper feedback data from the bad O2
    especially when the CE light is on, skewing the data.  I doubt the mixture is leaner.
    With the CE on, preset values are used to maintain the min mixture requirement.

    • 0 avatar

      It’s entirely possible that the O2 sensors were providing increasingly poor data until they failed, and the sudden increase in mileage is the result of returning to a preset map. However, it’s more probable the improvement is coincidence, or maybe even a lighter foot from having the CEL on.

    • 0 avatar
      Facebook User

      As the sensor wears out it starts to report leaner and leaner conditions (causing fuel trim to go up).  After the ECU has determined the sensor is beyond usable, it stops taking input from the bad sensor/reverts back to predetermined maps.

  • avatar
    John Horner

    Lean running is a sure fire way to burn valves and even pistons.
    Also, clearing the codes right before getting a smog check will not get you past most tests these days. There are a series of readiness codes stored by the car which all must be at the green light state before the OBDII emissions system checker will verify that there are no fault present. The idea of the readiness codes is to thwart just the kind of subterfuge xyzzy has in mind. They are supposed to ascertain that all emissions related systems of the car have been fully exercised and monitored since the last time the codes were manually cleared.

    • 0 avatar

      That OWT from the fixed jet carb days  just will not die.  Modern EFI systems are calibrated to run lean-of-peak except for WOT conditions.  Leaner is cooler, not hotter.  Back in the old days the “lean” that burned valves was actually slightly rich, and “normal” was very rich in order to cover up poor manifolding and weak ignition.

  • avatar
    cRacK hEaD aLLeY

    You are running in open loop. The ECM is ignoring the signal from the Oxygen sensor and calculating the air/fuel ratio based in part on inputs from the coolant and MAF sensors, but mostly the injection curves in your car are now a mirror of the factory table stored in your ECU. It is attempting to keep the fuel/air ratio as close to 14.7:1 as the engineers in their cubes envisioned depending on rpm and throttle position. It is not taking into consideration what’s actually going on combustion-wise.
    Your mileage increased which tells the other 02 sensors, pre and post cat, are clueless as far as what actually was happening in the engine. Just replace all 4.

  • avatar

    Here’s a followup, since I submitted the original question.  Since I submitted it, about 2 months ago, I’ve had the O2 sensor replaced.  Interestingly, I had misread the code’s description and it wasn’t the O2 sensor that was failing, but the O2 sensor heater circuit.  If I understand this correctly, that means the O2 sensor was really only inoperative for the first few minutes of each drive, until the car exhaust heated it up. 

    That said, I also learned that the heater circuit is actually part of the O2 sensor, so replacement of the sensor was required so it might as well have been failure of the sensor.

    I also learned in my research since then what others have pointed out, that I couldn’t clear the code and have the car inspected.  As for the improving mileage, since I submitted the question it had gone back to normal and I’ve also done a few paper and pencil calculations and found that the car is consistently getting about 5% worse mileage than the trip computer reports. So much for multiplying the reported tank mpg by the miles since fillup to find out how much gas I really have in the tank, since of course all trip computers are overly conservative on the range calculation.

    As for replacing the other three O2 sensors.  Welllll… I had this one replaced as part of the every 90,000 miles timing belt/water pump/belt replacement  which was a good bit of coin, so as long as the others aren’t failing I’ll stick with them until they start failing, or until some time passes after this last bill.

    • 0 avatar

      You are correct in that the heater does in fact help get the sensor warmed up more quickly, getting your car into closed loop operation faster.  While nobody likes car repair bills, sensors that old are really likely to be a bit “lazy” to say the least.  There is no way you car can be operation at peak efficiency with sensors this old.  Would you consider doing this yourself?  Check access; if you can get to the sensors easily, all you need is the socket that has a gap for the wire.  RockAuto has these sensors for your car for as little as $40 plus for Denso sensors…your car will thank you by returning the best performance…just my two cents.  I replace mine at 100K as a matter of course.  A little mechanical aptitude goes a long way to keeping older cars on the road for cheap!!

    • 0 avatar

      Only 2 sensors are used by the engine computer for mixture/tuning, the other 2, which would be after the catalytic convertors in the exhaust system, are just there to verify the catalyst is operating properly per OBD-II requirements.

      I Googled “P0161 LS400” and found that code to be for the Bank 2 Sensor 2 which would be the post cat. So this sensor actually has nothing to do with engine operation, other than indicating the catalytic convertor function has been compromised.

      You were running fine all along. But good to replace the part and clear out the code. I wouldn’t bother replacing the others.  Tossing new ones in is just car-guy feel-good stuff, nothing wrong with that.

    • 0 avatar

      I’ve seen the statement a couple of times that you can’t just clear the code and pass emissions.  Please don’t tell our 96 conversion van, as it doesn’t seem to realize this.
      Now, maybe the difference is the method of clearing.   I used a scangaugeII to do it.  Trouble code was something to do with bank 2 Cat.  Stayed gone long enough to get the emissions test done, and came back a day later.
      Of course, come february, I’ll have another data sample, as the 98 tacoma has popped a couple of times for O2 sensor.  But after clearing, it stays away for weeks or even months.

  • avatar
    Funk Forty Nine

    I’ve got an ’01 I30 with 47k miles.  At the rate I’m driving it I won’t live long enough to put a hundred thou on it.  My CE light came on recently (after the car sat for a month) but went out after a week or so.  It was on a couple years ago but Infiniti said there was nothing in there (my mechanic said it was an O2 code)  Should I replace the sensors?

    • 0 avatar
      Jeff Puthuff

      Are you certain that it’s an O2 sensor? Read this page on on how to retrieve the code. Trust me, you don’t want to replace an O2 sensor thinking that’s why the CE light is illuminated, only to have it illuminate again after the new sensors are in. I’ve got a ’97 I30 and the CE illuminated because the knock sensor was bad. It was a $30 DIY fix.

    • 0 avatar

      Don’t forget that the check engine light can sometimes be a bad gas cap, thanks Pepboys for checking that one for me!  (If I did more of my own work I’d get a code reader but I don’t have enough vehicles in the family right now to “loan” myself one while another one is out of commission while I work on it.)

  • avatar

    Another urban myth is that you can pass emissions on a OBD II car by clearing the code and disconnecting the battery.
    An OBD II reader is almost an essential tool box item nowadays. I have a cheap Harbor Freight Model (under $50) that I use often.  I am regularly asked by co-workers about problems their cars.
    Dealer and Independent service shops will occasionally suggest replacements of perfectly good parts. It those instances it pays to do some self diagnosis.

  • avatar

    I assume that this is the same xyzzy who has been writing in about other Lexus related questions.  Thank you for continuing to ask, it’s making me consider picking up a used 400 series Lexus and trying to make a “muscle sedan” out of it.  Forgive me, I was raised by a family of hot rodders from the deepest midwest farm country.  I’m in NM now but the spaces are just as wide open and begging to be driven through. 

  • avatar

    What would you do to make a muscle sedan out of it?
    I love mine just as it is, haven’t done any mods.  Effortless acceleration, good gas mileage for a 300hp V8 — 22-25 depending.   I’ve owned mine for 8 years and 133K miles and so far only two unscheduled repairs — the O2 sensor described here and a $200 headlight switch repair last year.
    As part of your due diligence on these, check on the timing belt status.  It needs to be replaced every 90K miles, so if you’re looking at one with, say, 175K miles, it  may be due soon.  It’s about an $800 job.

    • 0 avatar

      I know Lexi are supposed to be “whisper quiet” but I wish more manufactures would go back to chains for timing, at least they lasted forever.

      Muscle sedan, well how bout headers?  Cat back exaust?  Computer chip?  I’m not talking radical here, just a little more omp!  It already has dual exuasts right?  How bout Flowmaster mufflers for a little more growl.  Then with the miles you have I’d start checking the suspension bushings for wear and think about an upgrade. 

      I’d do all of this really slowly, I wouldn’t just throw my checkbook at the car, but then if I had a car like that (living in Northwest NM like I do) it would be for tearing up the interstates and the long state highways, I’d have a second tiny little 4-banger manual transmission car for commuting.  But to each his own, I’m a hot rodder at heart.  (My dad’s had a subscription to HOT ROD since he was 15 yrs old, and a boy has to read something while he’s in the restroom, lol.  It had quite an impression on me.  There’s a warning to all dads out there.  Be careful what your children get ahold of if they’re voracious readers.)

    • 0 avatar

      After driving mine for almost exactly 8 years (bought it in early December 2001),  my recommendation for the first modification, if you’re looking to make it more sporty, is a sportier suspension and tires.  I would not change mine, I love the effortless glide and wouldn’t want to ruin that, and I hated it when I had so-called sportier (i.e., noiser) Falken tires and I couldn’t wait to wear them out and get back to the Michelin MXVs (luckily with Falkens, it didn’t take long, heh).  But the car does get the leans in curves at speed and you can’t take corners fast.    

      Personally I wouldn’t change mine to make it noisier, but better cornering would be nice.  

      Not sure how much of a factor that would be in New Mexico though, if the roads are mainly straight.

    • 0 avatar

      Thank you for your suggestions, it is on my short list of rides to consider after getting my education leadership masters degree this spring, right after I get a principals job and start collecting the bigger paychecks.  I’ve been teaching for 10 years.
      I meant roads that are usually fairly empty of traffic once you get away from the population centers but then very hilly and curvy.  Think of all the Road Runner cartoons you watched as a kid, just at a little higher altitude.  (Gallup, NM 6,500 ft. above sea level, population 20,000; nearest city 137 miles.)

  • avatar

    Perhaps you all forgot about this Piston Slap:
    You all should know I got yo’ back.

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