I have had a check engine on a 2001 Subaru Legacy for some time now. Went to mechanic to fix few other things and he told me the oxygen sensor and catalytic converter needs to be replaced. The catalytic converter was installed by the small dealership I got the car from around January this year. Assuming the emissions fault code is not due to the catalytic converter, can the O2 sensor trigger such a code?

UPDATE I bought a BAFX Bluetooth OBD2 Scan Tool and tried it today. Although the check-engine light went away after the serpantine belt was replaced, I checked with the tool and got Fault Code P0420. And the O2 sensor bank 1 sensor 2 (picture attached) was showing 0.1V-0.3V. I cleared the codes and will see if it comes back in few days. Could the bad belt/wheelie trigger this error?

enter image description here enter image description here

  • Well, the mechanic that did my timing belt made notice that the O2 sensor needs to be replaces, and the catalytics converter as well. The cat was done withing the last year. So I am not sure what triggered the error code. I will replace the O2 sensor, reset the code, and will update here later on. Nov 4, 2014 at 19:40
  • Note that there's two cats and two sensors; you should double-check which are problematic.
    – jscs
    Nov 4, 2014 at 19:41
  • How can I check that? I asked the mechanic about which cat, he said that this one in my car is a full single piece. Nov 4, 2014 at 19:43
  • The diagnostic code will indicate which it is. You should be able to get it read for free at your local parts store. There should be, as far as I know, three sections to your exhaust: first section includes the front cat and sensor, second section the rear pair, and the last goes back to the muffler. Unless somebody did something weird: my Forester had a piece of pipe welded across what should have been the joint for the two rear sections when I got it, so when I needed to do the rear cat, I had to do the end pipe as well.
    – jscs
    Nov 4, 2014 at 19:45
  • 1
    Other things besides emission problems will turn on the CEL; without knowing what the code actually was, it's impossible to say why it turned on or off. (Also, it wasn't your timing belt that was replaced, as Paulster noted on your other question.)
    – jscs
    Nov 5, 2014 at 21:39

1 Answer 1


Short answer, yes - a faulty O2 can make the Check Engine Light / MIL to go on.

Do you know what the code is?

  • P0420. See update on the question. Nov 6, 2014 at 3:05
  • Yes, this code can trigger the check engine light
    – Nick
    Nov 6, 2014 at 13:03

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .