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1999 Subaru Legacy L: Wagon..the check engine light came..O2 sensor…

Customer Question
I have a 1999 Subaru...
I have a 1999 Subaru Legacy L Wagon with a 2.2 liter engine, the check engine light came on and when I scanned it it came up with code P0420 Catalytic Converter Threshold . My local mechanic says it could be the bank 1 downstream O2 sensor, as it is reading high in comparison to the upstream one, or he says it could be the converter itself, he says I should change both to be safe, that is about $ 600.00. Before I go dumping my Christmas cash into this car I want some better answers. It has a little over 90K on it.
Submitted: 13 years ago.Category: Subaru
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Answered in 6 minutes by:
12/4/2009
Subaru Mechanic: subie-tech, Subaru Technician replied 13 years ago
subie-tech
subie-tech, Subaru Technician
Category: Subaru
Satisfied Customers: 55
Experience: I am a Subaru Factory Trained technician and have been working on Subaru's for about 7 years.
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Normally this means you'll need to replace cat on a subaru. I have ran into this many times as I am a former master subaru tech. However, since normally when you replace the catalytic converter you also do the O2 sensor(s), I would recomend replacing the sensors first. Then replacing the cat if the code comes back.

If you need any more information, plese feel free to ask.
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Customer reply replied 13 years ago
OK, the downstream O2 sensor voltage reading is way higher than the upstream one and bounces all over the place. Would you suggest I start with the downstream one? The O2 sensors are $ 125.00/ea for Bosch one's?
Subaru Mechanic: subie-tech, Subaru Technician replied 13 years ago
Your A/F (air fuel ratio) sensor should fluctuate rapidly. Your O2 (catalist monitor) sensor should fluctuate at a slower rate. This is because the A/F (upstream) sensor is constantly adjusting the air fuel ratio according to the readings of the O2 (down stream) sensor. ...this is basically how it works.

I am not sure what your readings are, but the sensor closer to the cat (down stream) is the one you would want to replace. I would replace it with a Subaru part and not an aftermarket as the cost is compairable and the Subaru part is more reliable.
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subie-tech
subie-tech, Subaru Technician
Category: Subaru
Satisfied Customers: 55
Experience: I am a Subaru Factory Trained technician and have been working on Subaru's for about 7 years.
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