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There are a number of reason why one might want to bypass an O2 sensor, but it should be remembered that doing so will render a vehicle unable to pass an emissions test.
History
Mechanics were figuring out how to bypass O2 sensors practically from the minute Volvo introduced them in the 1970s.
Purpose
As the purpose of a rear O2 sensor is to monitor catalytic converter function, many people who remove or replace their converters with a non-stock unit find that doing so triggers a check engine light if the sensor is not bypassed.
Simulators
Because it produces its own voltage, an O2 sensor cannot actually be "bypassed." This term is used to refer to the installation of sensor simulators, which put out a false voltage to trick the computer into believing the converter is still functioning.
Cost
Oxygen sensor simulators can be purchased for between $30 and $50 from manufacturers like BBK, but must be specific to the vehicle.
Fun Fact
Some people have had success with installing a spacer to move the original sensor's probe a little further out of the exhaust stream. This causes the probe to run cooler, which simulates the effect of a working converter.
Source:
Dnd-automotive.com: How Does an O2 Sensor Work
Automotiveforums.com: O2 Sensor Bypass
Resource:
DIY O2 Simulator (Video)
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