So I was reading this Quora question which answers in the affirmative, but here's my misgiving: I was under the impression that the oxygen sensor which the ECU uses to adjust the fuel-to-air ratio, is located in the catalytic converter. I was also under the impression that without that sensor, the ECU is unable to operate, essentially breaking the engine by rendering it unable to regulate itself in accordance with the driver's interactions with the gas pedal.

Why am I wrong? The only answer I can think of is that there are other sensors elsewhere, and that the one in the catalytic converter is secondary and not essential.

  • Are you sure the O2 sensor is in the cat? Usually just before the cat or between the cat material and the joint. The cat replacements still have the boss for that sensor AFAIK.
    – Solar Mike
    May 14, 2017 at 20:48
  • Perhaps I'm mistaken; I thought it was. To be clear this is a theoretical question and is not referring to any particular car. I'm in the market, as it were. May 14, 2017 at 20:49
  • I am fairly sure that the O2 sensor is before the ceramic material that is the actual cat - the O2 sensor is usually in the metal body prior to that material.
    – Solar Mike
    May 14, 2017 at 20:51

3 Answers 3


Most vehicles which are OBD-II compliant (newer vehicles are a little different) use just the O2 sensors located before the cat to adjust the air/fuel ratios. The in-cat or after-cat O2 sensors are just used to check the efficiency of the cat itself. Therefore, in most modern vehicles a car can run just fine without a cat. Please note, this is not to say running without a cat is good for the environment. You will also get a trouble code for the cat itself which will appear on the dash (unless some type of tuning is done or a fake O2 circuit is used).

EDIT: (Just for thoroughness)

The forward O2 sensor(s) are usually located at the end of the exhaust manifold or just after the manifold in the exhaust pipe. This allows it to sense what's going on inside the exhaust stream at a point where all of the cylinders merge together, yet before it starts cooling off to a point where reading it will not work so well.

  • 1
    As a note, the ECU may put the engine into a "safe mode" if it doesn't have or doesn't like certain sensor readings, meaning that you'll get lower performance and/or efficiency. May 14, 2017 at 23:19
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    Additional note, at least in the united states, it is illegal to remove a catalytic converter from a car which had one equipped from the factory. Not that I've ever heard of it being enforced but...
    – Zshoulders
    May 15, 2017 at 2:58
  • The after catalyst oxygen sensor is used for fuel control in most fuel control systems. The catalyst acts as a mixture averaging device so the after catalyst mixture is smoother over time and therefore gives a better look at mixture control for adjustment of long term fuel trim than the pre-cat oxygen sensor. Newer fuel control systems rely on this even more than older systems. May 15, 2017 at 6:41

I've got 2 high mileage cars, both of which have cat removed because they were clogged. One of the cars has after-cat O2 sensor simulator, which prevents check engine light appearing on dashboard. Another car's check engine light comes on after each fuel up, I have OBD2 adapter and I reset warning light every time. I've put multiple years on these cars and they are running OK. Also it's much better to run car without cat than with cat clogged.

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    I'd caveat what you've said by saying "It's much better for the car, but not for the environment to run a car without a cat than with cat clogged." May 15, 2017 at 16:33
  • completely agree
    – oryades
    May 15, 2017 at 21:52

Actually, it can be better for the environment too. If a cat decreases emissions by 40%, and efficiency by 50%, it is less damaging to run without one entirely.

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