I have just purchased a replacement catalytic converter.

Using a flashlight, I can see the catalytic material inside the converter. When I was inspecting it, it looks like both the upstream and downstream o2 sensors are on the same side of the catalyst material inside. The exhaust gas would go past the upstream o2 sensor, then past the downstream sensor and then pass through the catalytic material, and then out the tailpipe. There is nothing at all between the 2 o2 sensor locations.

Is it possible that the catalytic converter part was welded in backwards?

It seems different than what I was expecting. I was expecting the upstream o2 sensor to be on one side of the catalyst and downstream o2 sensor to be on the other side so that they would measure the difference between the pre-converted exhaust gas and the post converted gas. I have attached a photo to help illustrate what I mean.

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Additional info:

  • 2007 Ford ranger
  • Engine: 4.0L v6 SOHC

It has a catalytic converter on each bank of cylinders. Each cat has an upstream and downstream sensor. This is the left (drivers) side.

  • 1
    Does not look right to me, return it.
    – Moab
    Oct 2, 2019 at 16:00
  • I take it you don't have the original to compare with?
    – HandyHowie
    Oct 3, 2019 at 6:58
  • Definitely looks like whoever assembled this is either insane or plain stupid. Return it.
    – Al_
    Oct 3, 2019 at 13:46
  • I have the original (on the truck) but it is oem and looks a bit different externally. I dont want to dismount it yet as I am still driving the vehicle.
    – TomE
    Oct 3, 2019 at 14:55
  • 1
    Here's how yours should look i.ebayimg.com/images/g/dSkAAOSwepJXbuPq/s-l300.jpg . Again, don't let it touch your truck.
    – Al_
    Oct 4, 2019 at 10:01

1 Answer 1


I suspect you diagnosed the situation correctly: that cat looks backwards. It’s a little hard to tell from the pictures but it looks like a straightforward manufacturing defect: the cylindrical section between the welds to the ball connectors appears to be reversed.

You might be able to get a muffler shop to cut that section loose, reverse it and re-weld it. Personally, I would just return it as a nonsense item.

By the way, it’s a good thing you noticed this situation. You could just as easily have put the whole unit on, had it pass emissions just fine and yet still had a check engine light from the downstream O2 sensor. That would have been frustrating in the extreme!

  • @TomE hopefully this one is what you want!
    – Bob Cross
    Oct 4, 2019 at 16:41

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