I gather that the upstream O2 sensor is the one right in the center of this picture. I also gather that the catalytic converter (the "cat") is the bulbous part of the exhaust right behind this sensor.


Question 1: Does the cat consist of just this one part or does it include the second 'bulb' in the exhaust that appears in this second picture?


Question 2: This third picture shows the section closest to the tail pipe. I can't spot the downstream O2 sensor. Can you suggest where I should be looking?


1 Answer 1


Picture 1 is the downstream 02 sensor. Upstream is located on the exhaust manifold.
Picture 1 contains both convertors.

Picture 2 is the exhaust resonator.

  • Ha! I was just typing that out and you beat me to it. +1 :o) Commented May 11, 2016 at 23:34
  • Let me just confirm. So the cat does consist of two parts, and the downstream O2 is located between the two parts. This means that measuring whether the cat is doing its job reflects only that the first part is active. It doesn't reflect whether the second part (downstream) is working properly. Is that right?
    – Calaf
    Commented May 11, 2016 at 23:57
  • @Calaf correct.
    – Ben
    Commented May 11, 2016 at 23:59
  • @Calaf: By "second part", you mean the resonator, right? A resonator doesn't really play an important role in emissions. It's primary role is to limit or adjust the exhaust note in addition to the mufflers.
    – Ellesedil
    Commented May 13, 2016 at 20:40
  • 1
    @Ellesedil - Most cats are actually two converters in one lump; one for CO/HC; one for NOx. If space is an issue, they can separate them like this without issue and it will still function as it was designed. IIRC, the NOx redux portion won't change the readings which the after cat O2 senses, thus why the sensor is between the two. The front one will be the CO/HC redux portion. Commented May 14, 2016 at 1:10

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