My check engine light comes on for a day or two then goes out for another day or two. Code says "inefficient catalytic converter". I replaced the whole exhaust system (both converters) about 4-5 years ago. The O2 sensors are the originals. Is there a way to know if they're working properly and which one is more than likely an issue (pre or post converter)? Thanks!

The front O2 sensor tells to ECU to make the mixture lean or rich depending on gas content. The rear O2 tells the ECU how well the CAT is working.
The fact that you get the message you get tells me that the rear sensor is working and the front sensor has a problem.
Of course, the problem could very well be that unmetered air is entering the intake manifold, which causes the ECU to enrich the combustion chamber with longer pulses from the injectors. Always check the vacuum lines, elbows, intake manifold gasket, injector seal leaks, etc.
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