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98 Honda Civic DX. The codes were accidentally cleared, but I am getting stumbles at acceleration at low speeds. It seems to be get worse with hot weather. Is this an indicator that the oxygen sensor is failing? If so, which one? I am thinking it's the one in the engine, which I believe is called the upstream oxygen sensor? Thank you.

 
Rep Power: 518

If you think the O2 sensor is causing it to stumble, then simply unplug it. (You are right about the upstream sensor there.)
See if it still stumbles. I bet it still does.

If it still does, then keep checking elsewhere. Ignition is the usual cause of a load related misfire. Cap/rotor, wires and plugs.....and the plug holes full of oil on many of the engines.

 
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It did turn out to be a distributor issue. Your advice right was on target. Thank you. I think the key was it did not throw a new CEL code after the old one was cleared.