Outline Of Diagnosis
WARNING: This page is about a different car, the 2013 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.
Detect that λ value remains low.
Judge as NG when lambda value is abnormal in accordance with λ value of front oxygen (A/F) sensor and running conditions such as vehicle speed, amount of intake air, engine coolant temperature, sub feedback control, etc.
λ value = Actual air fuel ratio/Theoretical air fuel ratio
λ > 1: Lean
λ < 1: Rich