cat reads from o2 sensor. Perhaps warranty the o2 sensors.
And if the Code still persists then follow the wire harness from the sensors
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I doubt the original o2 and the new o2 were both faulty and gave the same code. P0136 is out of range either too rich or too lean. Check fuel pressures, air leaks, maybe even a leaky injector. replacing the downpipe cat with another used cat is always risky... you never know the condition of it. if your out of ideas, find someone with a scanner that can do a real time data logger and see whats going on with the readings. You might have to bite the bullet on this one and pay someone to find out. better than throwing random parts at your car...

cel was on before
i replaced the OE down pipe cz the previous owner had a 2.5 -> 3" down pipe with a welded cat and it just sounds annoyingly loud.
thinking that the OE down would fix my CEL and cancel out the noise




any recommended shops?

maybe some of our sponsors can help you. i have heard great things about Speed Syndicate. Tommy at RG is also great. RG is building my FD so im bias toward them lol. if not, any aircare certified shops can help.

you should get a scanner and see what kind of readings u got on the o2 sensors, if the rear one is not producing any voltage, u might have a bad sensor or bad connector or bad wiring