Oxygen sensor Upstream or Downstream
 

Hi guys,

My 99 monty sport has one busted oxygen sensor out of 4, and my local mitsu dealer told me that it is the one on the passenger side after the muffler, can anyone tell me if this is UPSTREAM or DOWNSTREAM?

Many thanks,
Charlz

RE: Oxygen sensor Upstream or Downstream
 

I just looked this up on another forum...

The UPSTREAM 02 sensor is near the exhaust manifold.

The DOWNSTREAM 02 sensor is the one near/after the Catalytic Converter, which I believe is the one you want.

RE: Oxygen sensor Upstream or Downstream
 

They're located to provide a comparison of the exhaust before and after the catalytic convertor. So, yes - the one before the cat would be upstream, and the one after would be downstream.

Up Stream - Downstream oxygen sensors
 

I have a 2001 Montero. It looks really weird from beneath. It looks like I have 3 catalytic converters. One cat that is on the driver side that goes up into the engine; the other cat is on the very botton and lies perpendicular to the car- it is slightly on the passenger side; the first two cat intersect like a Y, connecting to what looks like a third cat-this one connects to the muffler. I can see the oxygen sensors, before and after the first two cats. I am trying to identify the rear cat on the rightside. That's another thing..all the sensors seem to be on the same side.

Which cat relates to bank 1 and which cat relates to back 2; I am trying to replace the rear O2 sensor because I am getting a P0421 code.

Bank 1 is on the passenger side, Bank 2 drivers

My car is 2003 Mitsubishi Lancer ES, and engine service light is on, code is also P0421.
How can I identify which one is upstream or downstream? Thanks.
Is it esay to replace them if I wanna do it by myself ?

Not sure what the Lancer set up looks like but 99% of the time this code means your cat converter is toast and your O2 sensor(s) are probably fine. Do a search in this forum for the codes P0421 (or P0431.) This has been covered in much detail before.

Ron

I ran across this article that I used in a prior post about the warm-up cat efficiency codes (431 and 421) This is from a tech article on a car without warm-up cats so the reference is to codes 430 and 420 but the testing sequence is the same. Note in particluar the sentence in red.

DTCs

P0420 Catalyst Efficiency below Threshold Bank 1

P0430 Catalyst Efficiency below Threshold Bank 2

Monitor Execution
Once per Driving Cycle. (On MIL equipped vehicles this test must be failed three times before the MIL light is switched on.)

Monitor Sequence
The following tests must be complete before the Catalyst Monitor can run:

HO2S monitor tests complete and no DTCs
No Fuel Trim DTCs
EGR Monitor Complete
No Misfire DTCs
No SAIR (Secondary Air Pump) stuck on DTCs (where fitted.)

It should be noted that faulty HO2S sensors do not produce the DTCs P0420 and P0430. The sensors must all be tested and shown to be reliable before the Catalyst Monitor can be called by the Executive.

I had both 431 and 421 episodes in the last year and both times it was pre-cat failure and all the O2 sensors were fine. The first time I took it to a dealer to verify this with their full system scanner and they said not to bother because 431 and 421 are always failed cats. And they were right.

Ron