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I understand that the voltage on the pre-cat O2 sensor must cross 0.5V (by a 0.3V margin). Is it also the case that the voltage on the post-cat sensor must cross 0.5V? The following image clarifies my question.

pre/post O2 sensors

As these two graphs show, the post-cat barely crosses 0.6V; it doesn't cross 0.5V. Is this an indication of a faulty post-cat O2 sensor?

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Your graphs look very normal. No, the post cat sensor will remain a nearly flat line when the engine is a steady speed, if the catalyst is good. It can and likely will lean or rich as the PCM chooses its fuel control strategy. So steady voltages of .2 to .8 are normal. The catalyst acts as a homogenizer of fuel mixture so the rich to lean cycling seen pre-cat gets averaged out.

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