without flow through, i can see why they would clog up after a few hundred miles.. the cat heats up and if the only path is to the o2 sensor. there is no where for it to go... maybe with a U and running past the o2 and back into the exhaust further down would last...


VW guys use tuning to turn off the secondary o2. the emission coding just asks about the o2 readiness for passing in most states (pa has some tough emissions laws too) and with the tune turning off the secondary and only running the primary, it will "pass"