DTMoss Posted June 20, 2012 Share Posted June 20, 2012 Hi everyone. I was told to replace my B1/S1 oxygen sensor. While trying to price replacement parts, the stores say "upstream" or "downstream". I know there are 4 oxygen sensors. Which ones are upstream and downstream? Thanks for the feedback. Quote Link to comment Share on other sites More sharing options...
WalkerTech Posted June 20, 2012 Share Posted June 20, 2012 DTMoss, I am a product specialist with Walker Products, a manufacturer and worldwide distributor of oxygen sensors. Your 94’ ES300 should only have three oxygen sensors, a post-cat front (Walker # 250-24298), a pre-cat front (Walker# 250-24360) and a pre-cat rear (Walker # 250-24360). Upstream refers to pre-cat, and downstream refers to post-cat. On your car, B1S1 is the pre-cat sensor that is closest to your windshield, between the engine and the firewall. Quote Link to comment Share on other sites More sharing options...
Lexlogic Posted June 21, 2012 Share Posted June 21, 2012 I am not sure if this Diagram can help where Oxygen Sensor are position but the downstream is located after the TWC. Quote Link to comment Share on other sites More sharing options...
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