slmaol

Hey,

I was going to replace my oxygen sensors so I called up OReillys to get a price. They were asking me if I wanted an upstream or downstream O2 sensor. What is the difference between them? How do I know which one I need?

1994 LS 400
200K +miles

Lexmex

Downstream O2 sensor usually refers to one after the main catalytic converter underneath the vehicle. However, if you are replacing the ones in the engine area, those are also called a/f (air/fuel sensors) and are upstream. If you're looking to fix gas mileage issues, then the a/f sensors are the ones you are looking to fix.

slmaol

Does downstream O2 sensor replacement refers to when vehicle hesitates or loses power ?

Lexmex

Upstream sensors are related to that issue. The downstream O2 sensor is purely an emissions device.

Looking up your vehicle with Denso, found the following:

Downstream part number: 234-2000

Upstream part number: 234-4211

python

iTrader: (4)

what code r u getting?

PureDrifter

iTrader: (10)

this .

SkulCracka

i just picked up a pair of the upstream denso's from rockauto.com

still need to install them but i have no ramps.

slmaol

Thanks for the replies - sorry for not getting back to the questions - that car issue is still not resolved and so the owener decided to do away with it. It was a 94 LS400 that was stalling at times, not starting sometimes and kept losing and regaining power....and no one was able to figure it out...so its gone. Thx ya'll

AYEmike

ignition coils lol