Quote:
Originally Posted by QuasiMondo
For everyday operation, not essential. If you want your check engine light to clear and pass your annual emissions inspection (provided your state has one), very essential. With newer cars (manufactured in 1996 and later) they were equipped with two oxygen sensors, one before the converter, and one after it. The first sensor works like any other oxygen sensor, monitoring the air/fuel ratio and making sure the mixture is optimal. The second sensor is used by the ecu to make sure the converter is actually working by comparing its signal output against the output of the first sensor. If the signal between the two sensors are a close match, then the ecu will assume that the catalytic converter is no longer operating and that is why your check engine light was triggered.
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I will have to disagree with you on some of your information. The catalytic converter is very important for every day operation, especially depending on what has actually failed. If the catalyst has melted, it will develop a restriction, that will cause excessive back-pressure in the exhaust system, resulting in severe power loss, and or excessive heat in the engine. Long term results could include major engine damage if the catalytic converter is not replaced.
As far as the pre-cat and post-cat sensors, you are right about the pre-cat sensor monitoring the oxygen content of the exhaust as it exits the manifold, and enters the exhaust stream. By measuring the oxygen content, the PCM can calculate how efficiently the engine is operating as far as air-fuel ratio. However the post-cat sensor also reads oxygen content coming out of the converter, thus measuring the efficiency of the catalyst, and its ability to store unburned hydrocarbons and convert it to the inert gasses for the exhaust. Oxygen sensors are just that, all they do is measure how much oxygen is in the gasses flowing past it. It can't measure anything else.