Your vehicle’s oxygen sensor monitors the air / fuel ratio. “Forging” an oxygen sensor refers to the process of bypassing the sensor so that the sensor does not transmit the correct information to the system computer. This is especially useful if you are trying to manually override the air and fuel settings, also called “trim,” or if you are trying to run your vehicle without a catalytic converter. If you are inexperienced in tuning your engine, you will want expert help. However, an oxygen sensor can be easily counterfeited by replacing the legitimate sensor with a fake sensor. Just keep in mind that this may or may not be legal in your state.