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I took my 2003 Toyota Matrix in to the dealership for service. It has 105,000 miles - the oxygen sensor needed replacing, but no other issues found. Now I feel as though I need to shift to a higher gear from 5th gear and that's the highest gear I have. The tachometer is at 3,000 at 70 mph, which seems normal, but I can feel the engine under my foot and I can hear it as well. It also feels somewhat sluggish at that speed, as though the car needs 6th gear. Any ideas?

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the oxygen sensor needed replacing

You may have identified the root of all of your issues. Depending on exactly what is happening with your faulty O2 sensor, it could affect any aspect of the intake-combustion-exhaust cycle. I would expect exactly the sort of symptoms that you cite; specifically, rough idle and reduced performance are to be expected.

  • On the plus side, replacing the O2 sensor should be really easy, a 5-minute job most likely. Make sure you pick up a special O2 sensor socket for removing the old one and tightening the new one -- it'll make it a lot easier than trying to get it off any other way. – R.. Feb 5 '13 at 2:14
  • @R.., I agree that the job is technically easy if you already know the procedure and have the tools. If you don't, there's no shame in paying a professional to take care of it. Mechanics gotta eat, too, you know. – Bob Cross Feb 5 '13 at 2:39
  • Actually I made the comment because this is a job that's easy even if you have no experience. The only tools you need are a socket wrench and an O2 sensor socket, and perhaps a breaker bar on the wrench if you're not super-strong. It's a good way for someone who's never fixed anything on a car before to get a first experience doing it and feel good about it. :-) – R.. Feb 5 '13 at 3:07
  • @R.., it's all good until they encounter the sensor in question is caked on the inside, they apply too much torque and it breaks, causing despair and anger in the person who's never fixed anything on a car before... :) – Enot Apr 17 at 22:55
  • @Enot for real. I’m about to replace an O2 sensor on a car with 200K miles. I’m going to be extra careful getting that sensor out of the cat! – Bob Cross Apr 18 at 18:37
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The oxygen sensor gives feedback to the control unit about how "appropriate" the amount of injected fuel for a given air amount sucked into the cylinders was. If it's faulty, the control unit typically applies some sort of heuristics ("guessing"), so two situations are likely to occur:

  • less fuel than optimal ("lean" mixture) - this means poor performance

  • more fuel than optimal ("rich" mixture) - means some fuel left over after combustion and reduced mileage

What's also important, leftover fuel in the exhaust gases can destroy the catalytic converter over time.

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