The first O2 sensor was introduced in 1976 on a Volvo 240. Your 1990 Toyota Pick-up truck should have at least one o2 sensor. click on the following link to learn more. https://www.google.com/search?q=1990+toyota+pickup+truck+o2+sensor+location&safe=off&biw=1170&bih=1030&tbm=isch&imgil=9Uqcaznb7Ji9YM%253A%253BGRkKWnF2c9oQ7M%253Bhttp%25253A%25252F%25252Fwww.toyotanation.com%25252Fforum%25252F102-1st-2nd-generation-1983-1986-1987-1991%25252F1028730-o2-sensor-91-lexus-non-ca.html&source=iu&pf=m&fir=9Uqcaznb7Ji9YM%253A%252CGRkKWnF2c9oQ7M%252C_&dpr=0.8&usg=__xuzM7m_BN3uWVZdmFiJpb9br0cs%3D&ved=0ahUKEwi3odSW3uHJAhXF9h4KHWfeBvsQyjcIJw&ei=1Q5yVvfiDcXte-e8m9gP#imgrc=9Uqcaznb7Ji9YM%3A&usg=__xuzM7m_BN3uWVZdmFiJpb9br0cs%3D

The first O2 sensor was introduced in 1976 on a Volvo 240. Your 1990 Toyota Pick-up truck should have at least one o2 sensor.

Here is an image to give you a general idea of location.