WalkerTech
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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.
