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Subaru catalytic converter problems

I own a 2000 Subaru Legacy wagon with 106,000 miles. In the past 8 months the CEL has been on nearly constantly, each time the code reads: “P0420. The PCM has determined that the catalyst system efficiency for bank 1 is below the threshold for the current engine operating conditions. (Bank 1 identifies the location for cylinder #1, while bank 2 identifies the cylinders on the opposite bank.)”

Since January, I’ve replaced the converter (the larger unit which houses two of the three converters) twice – once because of the CEL and the second time because my mechanic believed that the unit was “bad” when the light came on again. Two oxygen sensors have also been replaced to no avail. Are there other issues that could contribute to this code coming up? Any advice is greatly appreciated…

The codes only say what general area of the system the problem is in, not what parts are bad. A good mechanic will look at the signals the oxygen sensors are putting out with an oscilloscope. A bad gasket or a leaky (cracked or corroded) pipe that’s letting outside air into the exhaust can cause this code.