Oxygen Sensor (234-4030) by Denso®. Position: Upstream Rear. Make sure your engine runs efficiently and produces fewer emissions by replacing your malfunctioning fuel-ratio sensors with these top-notch products designed by Denso. Engineered with the use of the cutting-edge sensor technology, they are a fantastic way to regulate engine performance and emissions. Since they deliver OE quality and reliability, you can be confident in their superior fit and function in your vehicle.