Fuel injection can run without an o2 sensor, but it runs better with one. You can run into problems, as previously stated, if your f/i system was designed to have o2 sensors after the cat as well, because you won't have one to read after. It is actually illegal to install a cat on a vehicle not designed to have one from the factory due to the heat they emit, which place wrong could cause a fire. One o2 sensor per bank is the best way to go because it will read and adjust the air/fuel ratios more specifically with more information. They can be programmed to run without, but then you're simply hoping that the air/fuel ratio is correct, but not monitoring it and adjusting for temperature, load and other variables.
Devin
