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I have an 04 TSX that was showing a P0420 - Catalyst System Efficiency Below Threshold Bank 1 and the check engine light was on. I tried to pass smog and it failed of course. I changed the cat converter and the downstream sensor and the check engine light is still on. Any ideas what the problem is. Should I change the upstream sensor as well? Someone told me just use the engine cleaner fluid in gas tank, would this work? Thanks
 
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