Check engine light + vsc light on 2003 Toyota 4Runner

In 2003, I bought a 2003 V6 Sport Edition 4Runner. From 2004 through 2009, I received an annual P0156 code(Bank 2;Sensor 2 FAILURE), which resulted in replacement of the oxygen sensor that was behind the catalytic converter. In November of 2010, the P0156 code reoccurred. In response to this P0156 code, the dealer replaced the forward and rear oxygen sensor. This cost me over $400. On April 19, 2011, the P0156 code reoccurred again. Please assist. This issue is frustrating and costly(repairs and fuel).

Asked by for the 2003 Toyota 4Runner
I think you need to find an independent Toyota expert. I suspect wiring or connector problems or some contaminant getting on the sensor's exterior, like oil or coolant.
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