p0420 came up early of last month and end of april.

Read up on some old threads of rt cat.
-say ecu has some strict levels
http://www.obd-codes.com/p0420
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Symptoms

You will likely not notice any drivability problems, although there may be symptoms.
Causes

A code P0420 may mean that one or more of the following has happened:

* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings
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I'm running a highflow mangaflow cat 400 cells. On about 40,000 miles, i doubt its really the cat.

I reset the codes about a 2 weeks ago, anyone can say it was a false alarm?

 

Try an anti-fouler:
https://acurazine.com/forums/showthread.php?t=33074

 

i still haven't used an de-fouler on my i/h/c/e setup. putting two gaskets between my rt cat and greddy evo2 have done the trick. 40k on your aftermarket cat, i'd unbolt that thing just to look at conditions inside

 

wat do u guys think about the t1r racing converter at aj-racing? its a test pipe with a defouler already i think.