It's called a double D or shock absorber socket


It's the pre-cat sensors that are used to determine the mixture departing the engine, the post-cat ones are there to monitor cat function. Since the fuel trims adapt to maintain the value at the pre-cat sensor, it'll look good, if it's producing a fairly rational but off-calibrated to the lean side signal, even though the real mix is too rich, per the plug ...


In very simple terms the catalytic converter is an exhaust component that converts "bad" gasses into "good" gasses. The computer of the car monitors how well this functions by looking at oxygen sensors. Your car actually has two catalytic converters. The terminology goes as follows. When something is closer engine in the exhaust it is termed as upstream. ...

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