You should reach out to AA directly in that case. They guarantee No CEL and should be able to resolve your concerns. Any other modifications on top of downpipe? I have a protune with intake & charge pipe and still no CEL.
I also got a used titan and haven’t encountered any issues. I have put on close to 350 miles after installing titan.
I did notice on my protune that I had to switch connection a to c. For some reason, it wasn’t marked correctly.
I don’t get any errors with the titan protune. I have AA catted dp though.
Maybe remove the protune and see if you’re still getting an error for a few days?