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Hi
Need help I have few modes installed "MMR big pipe" air intake, down pipe, megnaflow exhaust and used SCT X4 to tuned with out of the box tune, P0420 Catalyst System Efficiency Below Threshold (Bank 1) keep coming back every other day after clearing the code, SCT told me there tune is doing what it spouse to and my O2 sensor or cat is bad or I don't have one installed? I don't know what to do next? my scan tool O2S11 reading are in mA when running they are in between -09 to +09 and I notice for 1st 30 minutes of drive graph fluctuation a lot then after 30 mins it fluctuate very little I don't know if this means anything, TIA
 

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So is your down pipe catted? Your problem is not a new one. There have been more than few who have posted issues with canned tunes and aftermarket down pipes (both with and without a cat). You may need to have a custom tune done.

U.M.
 
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