I have the same year, make and model truck and I have only one O2 Sensor. It is located in the exhaust pipe just after the exhaust manifold on the drivers side of the truck. I bought the duel Digital/anolog EFIE and only used the Anolog side. Wired it in just behind the glove box and mounted it in the ash tray door so to hide it when not wanting to see the LED light. I gained 3-4 miles Per Gallon and now get 21-25MPG. This is up from 15MPG when I got the truck two years ago. Increased to 20-21MPG with synthetic AMSOIL. Hope this helps in gaining MPG with an already worthy pick-up.
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For your 1990 Chevy Silverado K1500 you have 2 auto oxygen sensors that are upstream of the catalytic converter, one on each exhaust manifold. These are narrow band O2 sensors.
The quad digital EFIE is what you need for this vehicle. I don't know if you are planning on or have a MAP or MAf Sensor Enhancer but one of dual edge map sensor enhancers will work on this vehicle.
OR; you can try a hho chip that comes to you preset for specific year make and model.
We carry all of these products. Hope this helps and thank you for your inquiry.