Toyota Repair: 02 sensor warning, injector cleaner, fuel additive

Question
Ted, I have a 1990, V6, Toyota extra-cab pickup, w/5 speed & original clutch.  It has about 245k miles on it and runs perfect.  The check engine light came on the other day and the jumper "blink" check code was #21, the 02 sensor.  My mileage has been consistent at about 22-23 mpg.  The only thing I did different lately was add some injector cleaner at gas fill-up.  I could not clear the code by disconnecting the battery, so I may actually have a bad or failing 02 sensor. If this is the case, is it just a matter of replacing the 02 sensor?  If so, is it the 02 sensor located on the exhaust manifold or catalytic converter pipe or elsewhere?  And should replacing the 02 sensor otherwise cure the problem?  Or might there be other trouble involved?  Thank you,  Jerry.

Answer
This O2 sensor is located in the exhaust pipe just in front of the catalityc converter, the fuel additive could have caused this, you could drive it until the fuel with the additive is used up,fill the tank and drive it and reset the light a couple of times, if the light keeps coming back on then replace the sensor, keep in mind that any other malfunction such as a misfire or rough idle can also cause this code but it sounds like that's not the case.