Piston Slap: The Oxygen Network

Sajeev Mehta
by Sajeev Mehta

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.

[Send your queries to mehta@ttac.com]

Sajeev Mehta
Sajeev Mehta

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  • Japanese Buick Japanese Buick on Dec 03, 2009

    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.

    • See 2 previous
    • PrincipalDan PrincipalDan on Dec 03, 2009

      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.)

  • Sajeev Mehta Sajeev Mehta on Dec 03, 2009

    Perhaps you all forgot about this Piston Slap: http://www.thetruthaboutcars.com/piston-slap-fumble/

    You all should know I got yo' back.

  • RobbyG $100k+...for a Jeep. Are they selling these in fantasy land?Twin turbo V-6 paired to an 8-speed transmission. Yet still only gets 14mpg.Whatever money you think you would save over a V-8 will be spent 2-3x amount fixing these things when they blow up.
  • Alan Well the manufacturers are catching up with stocks. This means shortages of parts is reducing. Stocks are building around the world even Australia and last year had the most vehicles ever sold here.
  • Larry You neglected to mention that the 2024 Atlas has a US Government 5-Star Safety Rating.
  • Alan Why is it that Toyota and Nissan beat their large SUVs (Patrol/300 Series) with an ugly stick and say they are upmarket? Whilst they are beating the vehicles with an ugly stick they reduce the off road ability rather than improve it.As I've stated in previous comments you are far better off waiting for the Patrol to arrive than buy an overpriced vehicle.
  • Alan How many people do you see with a 4x4 running mud tyres? How many people do you see with a 4x4 running massive rims and low profile tyres? How many people have oversize mirrors for towing once in a blue moon? How many 4x4s do you see lifted? How many people care what tyres they run to save fuel? The most comfortable tyres are more or less the most economical.