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