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The heater in the O2 sensor may have failed you can measure the resistance in the sensor (two black wires) sensor disconnected should have about 4 to 6 ohms resistance if not sensor is bad. Make sure you identify the correct sensor before replacing it. Oxygen sensors are identified by bank and sensor position. Sensor 1 is always pre catalytic converter, sensor two is always post catalytic converter. If an engine is a 4 or 6 cylinder engine the whole engine is considered bank 1, if an engine is a V6, V8, V10....., the engine bank containing cylinder number 1 (first cylinder in the firing order) is considered Bank 1, the other bank is considered Bank 2. Interpretation Oxygen sensor designation B2S1 refers to Bank 2 Sensor 1, it will be the sensor on the engine side of the catalytic converter in bank two. Check this link to determine the cylinder that contains cylinder number 1. http://www.wellsve.com/custom_searches/ds_firing.php
There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online