So I recently asked this question as I was getting the check engine light.

Is obd code P0133 anything to worry about?

Was investigating doing it myself and changing the o2 sensor didn't seem like a hard task. However I'm not ready to search for leaks in exhaust etc which could also be causing the reading.

Some people advised taking it to a mechanic so I did so. He just reset the light and said "it was probably a glitch from when they did your clutch job. It shouldn't be coming back, but if it does call me".

I didn't feel comfortable with this especially where some answers said the light would come back or If I continued to drive my catalytic converter may get ruined.

Could this be just a glitch and he's advice is fair or do I need to investigate it again?

  • It is good advice.
    – Moab
    May 18, 2018 at 15:01
  • Better be safe than sorry is a good phrase : won’t hurt for you to make sure...
    – Solar Mike
    May 18, 2018 at 15:04
  • When the check engine light comes on, the first troubleshooting task I do is erase all the codes to see what pops again. Some codes which are read could be old codes. Clearing them out tells me where I should be looking. May 18, 2018 at 18:02

1 Answer 1


It is mostly lack of ignition that damages a catalytic converter, not oxygen sensor failure. I have driven with a car having a damaged oxygen sensor (resulting in a yellow engine light; I don't now remember if it was the front sensor or the rear sensor that was damaged), and catalytic converter didn't get damaged. It even passed the mandatory inspection afterwards!

The reasoning is that catalytic converters convert oxygen and oxygen-rich compounds (such as nitrogen oxide) with carbon and carbon-rich compounds (such as carbon monoxide) into carbon dioxide.

Now, in normal operation with oxygen sensor there is very little of both of these, some of which are considered pollutants. Whatever little pollutants there are will be mostly eliminated.

If the fuel mixture is rich, there are lots of carbon-rich compounds but little of oxygen-rich compounds. No damage done, as lack of oxygen slows down the reaction.

If the fuel mixture is lean, there are lots of oxygen-rich compounds but little carbon-rich compounds. No damage done, as the lack of carbon slows down the reaction.

If you have a faulty oxygen sensor, the worst that could happen is that you have either too rich or too lean mixture. Neither should damage the catalytic converter irrevocably (except perhaps alternating lean-rich mixture could cause potential damage, if the carbon-rich compounds stay in the catalytic converter semi-permanently and then the oxygen-rich compounds arrive to allow the chemical reaction to proceed -- now, could this happen if your sensor is damaged, I'm not sure, perhaps somebody more experienced in cars could answer).

Now, consider instead what happens if you have lack of ignition. Lots of carbon-rich compounds will be injected into the airstream having lots of oxygen-rich compounds. No combustion in the cylinder due to lack of ignition. Everything goes to the catalytic converter that is very hot, and thus, there is a huge reaction rate in the already hot catalytic converter producing even more heat. The heat damages the catalytic converter.

I would ignore the code for now, but if it repeats, you may need to investigate it further. Damaged sensor or wiring? Exhaust leak? Usually, there is a reason for most codes and the times where the error light would light up randomly and then automatically turn off are past us.

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