My check engine light came on and gave me reading 21 for oxygen sensor replacement or malfunction. But in doing some reading, Toyota warns that there may be more than one 02 sensor to replace! So which one? Also, how can I tell if it is "before catalytic converter", or "after" part that I need? (Does this mean in proximity to the converter--for and aft?). Just trying to save some money and do it myself, but dont want to order wrong parts over net for discount and make a mistake.
Well, BEFORE or AFTER is pretty easy to figure out. The exhaust flows from the engine to the rear bumper, with the Cat somewhere in between -- hugely oversimplified.
The O2 Sensor(s) are located in the exhaust pipe/manifold. There will be one right after the manifold comes out of the block, and maybe another one right after the exhaust pipe comes out of the CAT. The CAT will be located between the engine and the muffler. The first sensor decides if the fuel load is burning well, the second sensor decides if the CAT is doing its job. The second sensor is generally not a problem. The first one is more problematic, and it most likely the source of your Code. I don't think you have the second O2 Sensor.
The incorrect term for these -- but one that seems to work -- is Upstream and Downstream. The Upstream is the one nearest the engine -- before the CAT in terms of which way the exhaust flows -- and the Downstream is the one after the CAT, if it exists.
Replacing the O2 Sensor is much the same as replacing a spark plug, you will need a special socket wrench that can accomodate the wire connected to the sensor -- this socket typically costs in the range of about $10, and fits every sensor I've ever come across.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.