The O2 sensor is located on the downpipe of the MR2, although you were not specific to which MR2 generation/model you are referring to. Most are located there. The downpipe will be bolted to the exhaust manifold, which is bolted to the side of the motor closest to the firewall.
Where is the O2 sensor on 1993 Toyota Mr2
Follow this link to Toyota Technical Service Buliletin for your answer. http://www.toyotapart.com/ENGINE_BANK_1_AND_BANK_2_A_F_AND_02_IDENTIFICATION_T-EG034-07.pdf
Follow this link to Toyota Technical Service Buliletin for your answer. http://www.toyotapart.com/ENGINE_BANK_1_AND_BANK_2_A_F_AND_02_IDENTIFICATION_T-EG034-07.pdf
that is the oxygen sensor on the exhause manifold.
The O2 sensor on a Toyota Corolla is located on the exhaust manifold. The O2 sensor properly reads the air fuel ratio to tell the engine how much fuel to use.
It is on the engine, under the intake manifold, there should be a wire going to a sensor there. It is right in the middle.
For the 1993 Toyota Tercel, the O2 sensor has three different wire colors. These colors are: black, blue, and white.
See related link which includes instructions for testing any O2 sensor.
The Toyota mr2
The one closest to the exhaust manifold is the upstream O2 sensor.
O2 sensor
Its included in the following instructions for changing out the post cat O2 sensor. http://www.carspace.com/guides/Toyota-Echo-How-to-change-post-catalytic-O2-Sensor