So my blazer is once again throwing a P0420 code: Catalyst System Efficiency Below Threshold


Last time it threw this code, I replaced the fuel pump and the light turned off for a few months and just recently turned on again. It seems as if the car is idling just a bit rough as I can feel some slight shaking in the vehicle.

Is it possible that a bad fuel pump (or bad fuel pressure period) can cause this code to register??

 

when I got the P420 code it was my pre cat it was clogged I would bet that is where your problem is you should have a pre cat at the y pipe then yore regular catalytic it will be one of these as I said I would bet on the pre cat being shot

 

How can you tell which one is the problem? Is there any type of test?
Also, if i have to order a new cat...which one do I order? I see two different types: first one is rectangular shaped with an inlet/outlet on each side like this

second one is the same thing but has what looks to be about a foot or two of piping coming out of the sides. like this

 

a universal direct fit is usually a safe bet, did you look to see if you also have the pre cat? it is smaller and just before the big one at the y.. the brand you get will be your choice depending on where you live, but if you have the pre cat you will have to determine which one went bad tap with a hammer to see if any rattles or get a hold of a
temerature gun to measure the heat at both ends there are videos on how to do that look on youtube or eric the car guy