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2000 VW Jetta Passing Smog Test with Bad Catalytic Convertor

I just bought a used car (VW jetta 2.0) with 76,000 miles. It passed smog test before I purchased it. However I drove it 80 miles (up a hill for the last 5 miles) and the engine light went on. A mechanic scanned the OBD and told me the catalytic convertor is bad and possibly the rear 02 sensor as well. I'm guessing the car should not have passed smog. Note that the Smog report also showed 0.0 measured % of 02 at both test speeds,yet showed Pass. Why 0.0? How could this be unless the sensor was unplugged?

Answer:

0.0% oxygen content is actually very good IF all other emission numbers are within their averages. It means your VW Jetta used up all it's oxygen during the combustion and catalytic operation processes. If NOx was high, CO very low, and O2 low as well, it might indicate a lean fuel condition. Neither says your VW Jetta's catalytic converter is defective however. The trouble code is a different story. If your Jetta's check engine light is on and the trouble code indicated a CAT Efficiency Code we'd recommend first inspecting the rear O2 sensor and then the catalytic converter. A "lazy" rear O2 sensor will trigger a CAT Efficiency trouble code without the CAT actually having a problem. Given the cost of replacing the CAT as opposed to the Oxygen Sensor, we recommend making very certain which component is actually defective. As far as the rear (or front) O2 sensor being disconnected during the smog check... If either O2 sensor was unplugged during the smog check the check engine light would have turned on instantly and your Jetta would have no chance of passing the smog check. posted by SmogTips Support

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