Quote:
A friend of mine purchased an O2 sensor resistor that he added to his car and he swears he gets better gas mileage with the resistor
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Measure the value of his resistor. Get a resistor of the same value for a penny and try it yourself. I think it must be a pretty high value resistor to make a difference, because the input resistance of the ECU must be very high. Now if you are talking about injecting a voltage into the O2 sensor wire, that is something else.