The 2004 Toyota 4Runner Bank 1 Sensor 1 is situated on the ventilation system before the exhaust system. It is related with chambers 1，3,and 5 (incl' 7 for the 4.7L V8 motor) as they are each of the a piece of Bank 1. Sensor 1 is the upstream O2 sensor that actions the fuel to air proportion emerging from the ventilation system. The voltage signals are then shipped off the PCM to direct the blend. In the event that the sensor starts bombing You might see unfortunate gas mileage,disappointment of the emanations test,harsh standing by,and a lit Check Engine Light. The assessed substitution cost is around $260 to $300 for parts and $90 to $115 for work.