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The catalyst moniter is the rear oxygen sensor located after the catlytic converter. It sole job is to moniter whether the cat is performing corectly. It does this by comparing it's gas readings to the front (before cat) oxygen sensor readings. If it sees readings too similar to that of the front sensor, the vehicles computer(PCM) concludes that the cats are not changing the exaust gases thus are not doing their job. A catalyst moniter code can be set for 2 reasons. 1) that the catalyst moniter has failed (not typical) or 2) that the catalytic converter has lost it's catalystic properties, and is no longer working. This latter is usually the case, and requires the replacement of the catalytic converter. You can diagnose a failed cat with a scan tool that is equipped with scan display data. You can watch the voltages from the front and rear O2 sensors, after fully warming up the engine, and see if they follow each other too closely. It will be appearant, because if the catalytic converter is working properly, the rear O2 sensor will switch will have very little fluctuation in voltage, while the front will be constantly switching from near o volts to nearly a volt, several times a minute.

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Q: Check engine light: Reading stated catalyst effeciency below threshold?
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