history:
Last year in August I replaced all the fuel lines, fuel filter due to error codes. I also replace all of the vacuum lines and elbows due to error codes. At the same time I also replaced the flex pipe on the manifold unit. Then I replaced both the front oxygen sensor and rear oxygen sensor only to find out that the cat. converter was dead. So, I got a new cat for the car as well.

Since august I put about 3800 miles on per month and now I just started to get the po422 code. Why? What failed? What should I replace now? Could oxygen sensors and/or a cat die that quickly? Or is there something more serious that I'm missing?
The car is an '96 Volvo 850 GLT Wagon with just under 230k on it now.
Thanks.

A P0422 is an indication that the rear O2 sensor finds the exhaust dirty. If your MPG is low, suspect another vacuum leak like the hose from the air filter to the throttle body.

Because this is a replacement CAT and probably not OEM, consider getting a stand-off adapter for the rear sensor. This backs out the O2 sensor from the mainstream flow and reduces error messages, used for high flow exhaust systems.

Currently getting 29 mpg average and have been for several months now. I'll check the vacuum lines again and see what I find.
The cat was a complete unit from the manifold to the tail pipe of the exhaust.
What is a stand-off adapter? Not sure what you mean here. More info or a link to what you are talking about would be greatly appreciated. Thanks.

http://www.ipdusa.com/products/8656/109553-oxygen-sensor-spacer-cel-boss

This is what I have for a cat. converter.

DAVICO CONVERTERS
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These converters are equipped with more surface area which means better performance and quality

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They have more Platinum and Cerium

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Cerium absorbs oxygen, which promotes cleaner burning that is also important in triggering the engine light on more sensitive obd II systems.

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Commitment to Quality

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Has the widest catalytic converter coverage in the industry- including new widely expanded domestic coverage.

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Our converters are EXACT™ fit. No kits and no so-called direct fit that require cutting, sliding, clamping, or welding for installation.

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Attention to detail. Direct fitting flanges and air tube connections make our product fit more like OEM than any other which results in less returns and a more satisfied installer.