My 2000 Si is throwing code 67. Catalyst system efficiency below threshold, and the probable causes are the cat or the secondary o2 sensor. The car has 93,000 miles on it. I don't really hear about OEM cats dying. Is it most likely the cat or the o2 sensor. I just have a hard time believing that is is the cat, as the car has never ran rich.

 

its throwing that cold because you are either running too rich or something to that nature, or you probably popped your cat out.

to remedy this code, get an o2 simulator

 

Your cat could be loose.

 

The code 67 is 2 trip detection so it must see the catalyst inefficiency in 2 trips to set the CEL. The ECU want to see that the O2 level aft of the cat is lower then ahead of the cat. This means that it's functioning as a catalyst and oxidizing the exhaust gases.

If the 2-O2 sensors read the same then it means the cat is not doing it job and it'll set the code if it see's it 2 times. It only checks the function of the cat 1 time per key cycle if your driving cycle meets the criteria of a trip.

So what can cause this code 67?
1) Non functioning cat
2) Hole in the cat
3) Faulty O2 sensor(s)
4) Hole or leak in the exhaust, donut gasket between the cat and the a-pipe.

Quick test to make sure the cat is really working, after a good long trip measure the temperature of the exhaust pipe before and after the cat. If the cat is funtioning the temperature after the cat should be 100deg C higher than before the cat.