I backed up in a dirt lot and heard a metal scraping sound. After driving about 30 miles, the check-engine light came on with code P0420 indicating "Catalyst Efficiency Below Threshold (Bank 1)". I inspected the bottom of the car and found a scrape mark on a section of an exhaust pipe. There is soot deposited near a presumably loosened connection of exhaust pipes. The possible reasons for P0420 are:

  1. Bad O2 sensor
  2. Intake vacuum leak
  3. Bad spark plug
  4. Bad catalytic converter

With the car running well, I don't think it is due to reason 2 or 3--I hope it's not reason 4. Can I measure upstream and downstream O2 sensors using a voltmeter? If I could, what are the normal functioning upstream and downstream O2 sensors' values? Thank you to whoever took time reading/answering my question.



  • "Can I measure upstream and downstream O2 sensors using a volt meter? I" probably not most mechanics use an oscilloscope. Check the O2 sensor right after the converter, see if it is damaged.
    – Moab
    Jul 7, 2020 at 0:10
  • Where is the exhaust joint (2nd picture) in relation to the catalytic converter? Jul 7, 2020 at 11:03
  • It's at the downstream, after the cat leading to the muffler.
    – Winnie Law
    Jul 7, 2020 at 15:13
  • I will try to obtain an oscilloscope next and check the O2 sensor right after the converter as suggested and update here when it's done. Thanks!
    – Winnie Law
    Jul 7, 2020 at 18:43

1 Answer 1


The upstream O2 sensor should rapidly change between 0 and 0.5 volts. The downstream should hover at about 0.5V constantly.

I have seen this issue with a relative's 2009 Fit Sport. Unless you live somewhere very anal about emissions testing, I wouldn't worry too much about it. Now, if the catalytic converter were to clog (and cause a poor running condition along with it,) I would worry about it. If you have a laser thermometer, the downstream or "out" side of the converter should be significantly hotter than the upstream side whilst running.

Anything occurring AFTER the downstream O2 sensor should not have a notable effect on performance or safety (besides the danger of inhaling carbon monoxide.)

If you are mechanically inclined, it wouldn't hurt to remove the catalytic converter and inspect in the inner mesh material. It should be fully intact and you should be able to easily see through it. It would also be a good idea to inspect that black mark (exhaust leak - perhaps a ruptured donut gasket.)

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