On a 2005 Hyundai Santa Fe, I have a P0430 code. The description is "Catalytic converter below efficiency, bank 2". How many O2 sensors are there and where are they? Also which one(s) could contribute to this code. Thinking I'll try the sensors first before having the catalytic converter replaced.
Typically, modern cars have two O2 sensors (upstream and downstream) per bank. Your code indicates that you have at least two banks. I don't really believe your engine would have more than two banks, so two banks it is. Thus, there are 4 sensors: downstream bank 1, upstream bank 1, downstream bank 2 and upstream bank 2.
The two sensors per bank are used because they can identify reduced catalyst efficiency and throw a problem code, exactly like you have now. One of the sensors is used to control the engine in a closed loop mode, and the other is to monitor the catalytic converter process.
To the question you didn't ask: your error code may in theory be due to O2 sensor fault, but it may unfortunately mean you need to replace an expensive catalytic converter. Before doing so, rule out bad O2 sensors and exhaust leaks as the cause.
In your case, you need to be concerned with only the bank 2 sensors, and there are two of them. For identifying which bank is bank 1 and which is bank 2, see these links:
...so unfortunately, it appears there is no generic answer and it may vary from car to car. So, you may need the service manual for your car for identifying the bank 2. Usually, the forwardmost cylinder would be cylinder 1, and thus belong to bank 1.