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Check engine light with P0420 code on a 2001 Forester L

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My son's car has been displaying this since the spring.  Currently, he is out of the area on a summer internship.  Yesterday, he took the car to a local mechanic, who stated that the front O2 sensor is good, so the code is probably originating from the rear O2 sensor or the cat. My son will be returning home in two weeks, so I want to have it repaired.

My question is that if it is the cat, would an aftermarket cat be a good choice?  I believe the Subaru cat is about $1000.  The car is in good shape other wise and has about 90,000 miles on it.  

Thanks.

Could be an exhaust leak. When was the last tune up, Plugs, wires, pcv, fuel filter  and  air filter? If it is the cat Rock auto has them for $230 plus shipping. EASTERN CATALYTIC # 40237 ECO III Direct Fit

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Two local mechanics said that is probably the Cat.  How is the quality of the Eastern Cat?  One guy was saying that it depends on the amount of precious metals that they contain.  That is, the more precious metals, the better the quality.

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I plan to.  I am either going to take it to the local dealer or a guy that specializes in Japanese cars.

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