If the downstream converter is restricted, a universal converter should be no problem at all. The upstream cat is smaller and it helps the downstream cat store more oxygen so it can operate more efficiently. A universal cat in the upstream location would be overkill. Keep in mind that converters don't plug up on their own. They plug up as a result of an engine performance issue, usually running too rich. Make sure the problem is taken care of BEFORE you replace the converter, or the new one will do the same thing.

 

They're both stock, with 226k miles on them. Think it could just be old age?

Plus, it does have the stage 1 NA heads, the 75mm ported TB and it's tuned for 89 octane (I've been running 93)

 

Sure they're old, but that doesn't necessarily mean they're plugged up, or they're not doing their job. I'm old, and I still do my job, I get plugged up now and then but that's a different story


If you do a back pressure check, that will tell you if it's restricted. The PCM will tell you if it's not doing it's job, by setting the dreaded P0420.

 

It mainly only runs warmer on the interstate. In town, the difference is barely noticeable. Hasn't set the code yet, but the warmer temps and lower gas mileage tell me one is maybe partially clogged.

For what it's worth, when I bought it, it had a bad o2 sensor (pre cat, passengers side) and mileage was terrible until I replaced it. Think running it with a bad o2 sensor could start clogging one?

 

Absolutely it can, and it will! When oxygen sensors get old, their response time slows down, and their voltage output goes down. It's normal, it's the way it is, and that's why it's recommended to replace them at 100K miles. When voltage output goes down, the PCM interprets it as a lean fuel mixture, so it adds fuel, that the engine doesn't need, and the converters can't handle.... double whammie!

 

So it's likely that one is clogged. Is there any easy way to determine which one is bad?

 

You can make a tester very easily, and not spend a ton of money. You'll need an 18mm spark plug anti fouler, a 6 or 8 foot piece of hose, and a 0-10 or 0-15psi pressure gauge. Connect one end of the hose to the anti-fouler, and the other end to the gauge. Remove an upstream oxygen sensor and screw the anti-fouler in its place. Engine at idle: rule of thumb is less than 1psi is acceptable. Quickly snap the throttle open and back to idle: should not exceed 5psi during the snap.

 

I knew the test, I was wondering if there was a way to see which one was clogged.

 

If you come up with a "fail" on that test, you will need to create an "access hole" between the two converters, and test it there. The last test is install the tester in the downstream oxygen sensor bung. If that comes up too high, the muffler etc is plugged. It's just a matter of finding the area that is causing the pressure increase.


EDIT: The exhaust systems on these vehicles have very low backpressure, (typically less than .5 psi) that's why headers and high flow stuff doesn't do much for them.

 

Hopefully it's not the muffler, that's less than a year old.