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I don't like the idea of replacing the cat, especially with a cheapo unit that is sure to be junk.
The post-cat O2 sensor shouldn't affect fuel economy anyhow. It merely monitors cat efficiency, and isn't used to adjust fueling rates on the vehicle. The primary (upstream O2) sensor is used to fine-tune the A/F ratio.
Is it a P0420 code? How fast do you drive on the freeway?
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