Oxygen sensors

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"Upstream" oxygen sensors are in front of the catalytic converters. Those are the first ones the exhaust gas hits, and provide the feedback signals to the Engine Computer to tell it if the fuel/air mixture is switching between "rich" and "lean" properly, about two times per second. The computer tweaks the mixture slightly based on these readings.

"Downstream" oxygen sensors are after the catalytic converters. They are the last thing the exhaust gas sees before going out through the mufflers and tail pipes. Their only purpose is so the Engine Computer can monitor whether the converters are working properly. If they are, the readings might change between "rich" and "lean" once per minute or two. If no change takes place in the composition of the exhaust gas inside the converter, the rear sensor will switch between "rich" and "lean" at the same very fast rate as the front sensor. That is how the computer knows the converter is not cleaning up the exhaust.

The rear O2 sensors have absolutely no effect on engine performance, and the front ones have too little effect to be easily noticed. The exception is if one of their internal heaters were to short. That could cause a fuse to blow, then the symptom typically is the engine cranks but will not start or run. It is more common for a wiring harness to the oxygen sensors to fall down onto hot exhaust pipes, then melt and short to ground.

Also, be aware there are dozens of potential fault codes that could be set related to oxygen sensors, and they mean very different things. Most people replace an O2 sensor for a fault code like, "running lean too long", but that code is set by a properly-working sensor reporting an unacceptable condition. Replacing the sensor doesn't fix the problem. If the fault code is "catalytic converter efficiency", that is also detected by watching the signals from properly-working sensors. The converter needs to be replaced to solve that, not the sensor.

The fault codes will also specify which side of the engine has the defect. The driver's side is "bank 1", and the passenger side is "bank 2". The bank will be included in the fault code description.