where are the O2 sensors and what is the funtion

Hello family-- The oxygen (O2) sensor monitors the air-fuel mixture of your engine by measuring the amount of oxygen in the exhaust gas. Your car will run best with the right air-fuel mixture and the O2 sensor sends a message to your fuel injectors whenever it detects the wrong mixture. I will send you to a diagram of the location of those sensors on your 1991 Toyota Previa. Just click on the diagram for a full-sized view. Go to-- http://autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/22/c4/81/0900823d8022c481.jsp. I hopes this helps you. Good luck from Bovan.