This code appeared. I put in a new after market lambda sensor and cleared the ECU. After driving a while it returned ned. The o2 sensor bank 1 sensor 2 reading was flat line on 0.8 v. Then a new genuine lamda sensor was put in. P0138 code returned. Now the bank 1 sensor 2 reading is not on flat. line on it does move up and down. Any idea what is causing p0138 code to return? 
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