I know High Flow cats tend to throw the P0420 code but with my car at 131k miles I am not sure if it is something else throwing the code like the 02 sensors or there wires. I have had my rallisport racing v1 UEL header w/ highflow cat for 2 years now. First year was CEL free. After the first CEL it has been coming on sooner and sooner. I now can only go about 250miles after clearing the code till it pops up again. It also started to pop up around town, as before it was just triggered on the highway. My Question is should i replace my front and rear o2sensor's or at least have them checked(how much for just a test?) Gas mileage seems normal (mixed driving):22.7 mpg's summer, 21 mpg's now in the winter. With my car being at 131k and needing maintenance here and there i don't want to ignore the P0420 that keeps coming up & just assuming its the HFC nor do i want to ignore the CEL if a different code pops up under it not knowing a different code was triggered. If it is just the cat should i get the spacer for the rear or will that hurt my cars a/f ratio? emission check in Mass. is just read by ObII. Thanks for any help and reading another P0420 thread!