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Lots of causes to this.

First make sure spark plugs, wires (turbo may be coil-on-plug), air and fuel filters, and PCV valve are all new.

 

Next, get a scanner or look into getting a program like Romraider so you can watch and record live data.

 

Read through some of Cardocs P0420 diagnostic thread on SubaruOutback.org. Tons of info there about those code and what can cause it. But the key to diagnosis is to find out what the computer is seeing from sensor inputs and how it's responding by altering fuel trim.

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