O2 sensor

got one bad, anyway to tell "upstream" from down. Or should I bit the bullet and do all 4

 

Scan it, get the code and it should tell you which one it is.

 

The code will give you a bank number.
I will say, just because it’s a code for the sensor, doesn’t mean the sensor is bad.

B1 = Passenger Side
B2 = Driver Side

S1 = Closest to the exhaust manifold (Front)
S2 = Farthest from exhaust manifold (Rear)

 

TNShooter gave a straight/to-the-point explanation.

Here's a chart that covers a number of Toyota models:

http://alflash.com.ua/Learn/banksengine10.pdf

 

thanks, off to the shop, scaner in hand.