Bliath, In closed loop, the O2 sensors are used to adjust the fuel mixture. This type of O2 sensor is essentially a switch, it will read a voltage <300mv if lean and >600mv if rich. The actual voltage is an unreliable indicator of how rich or how lean. So the strategy used in closed loop is to adjust the mixture very slightly rich and wait for the O2 sensor to register a high voltage, then immediately after that lower the injection time and watch it switch to low voltage. It switches frequently, a couple of times a second. This switching also has the benefit of feeding the cat a very slightly rich (HC) and very slightly lean (O2) alternating mix so the cat stays warmed up and efficient.

There are two sets of O2 sensors for each bank, the passenger bank (bank 1) and the driver bank (bank 2). The pre-cat sensor is in the exhaust manifold before the cat, this is sensor 1. It is used for fuel control. The post-cat sensor, sensor 2, is behind the cat and is known as the catalyst monitor. The cat should take the rich/lean mixture that keeps the pre-cat O2 sensor switching low/high and even them out by burning the HC/O2 mixture, so the post-cat sensor shouldn't switch but see an even stream. If the post-cat sensor switches like the front one, it sets a catalyst inefficiency code.