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Assuming we are talking about the listed 99 Civic- Bank one would be the O2 sensor before the Catalytic converter, while bank 2 is after the Catalytic converter. The bank one sensor sees fluctuating oxygen levels and adjust fuel to match. Bank two is just checking the function of the Catalytic converter so it see very little change. If you are getting so much fluctuation that the ECU throws a warning light, there may be a crack in the manifold near the bank one O2 sensor.
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