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Posted

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.

Posted

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.

Posted

I am not sure if this Diagram can help where Oxygen Sensor are position but the downstream is located after the TWC.

post-26724-0-80582000-1340253722_thumb.j

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