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Upstream o2 sensor voltage

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2008 Altima 2.5L. Live data voltage show fluctuations from 2.0 to 2.3. Intermittent P0420 code. Downstream sensor shows steady at .480. Bad upstream sensor? Cat temps before and after show the cat to be operating correctly.
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Those upstream (A/F) readings are pretty normal, it's a 2.2V wideband where lower V is rich and higher V is lean. If the downstream sensor isn't swinging at a highway cruise then it's either because the cat isn't lit yet (not fully warm) or ECM has given up on the cat. It looks at voltage to gauge the health of the sensor, but it looks at swing time to gauge the health of the cat. The rear O2 readings (and for that matter, the cat temperatures) don't really mean much at idle. P0420 by itself almost always means a dead or dying cat, if something upstream is causing it then other codes will usually be present.
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