My CEL is on and the code reads P0420 on a 2003 Vibe.

There are apparently three options:

  1. Pre-cat O2 sensor
  2. Post-cat O2 sensor
  3. CAT model 51274

This is most likely beyond my mechanical abilities (been bitten), but I'd like to get the part from a reputed maker (been bitten there too), and take it to my mechanic. Candidate parts appear to be:

  1. AB Catalytic 51274.5 Catalytic Converter
  2. BOSCH O2 Oxygen Sensor Pontiac Vibe GT 1.8L w/ UNIVERSAL WIRING to 1-4-wire sensor

Am I on the right track? How can I determine whether it's the pre- or the post- O2 sensor? Does it make sense to replace one of the O2 sensors first, and replace the cat if that doesn't turn the CEL off?

  • @Edward, yup, missed that.
    – Moab
    May 11, 2016 at 18:20
  • I would go with the upstream 02 sensor- that is one that controls the air /fuel ratio. The downstream 02 monitors the condition of the cat and usually doesn't trigger a CEL unless the cat is bad.
    – Old_Fossil
    May 11, 2016 at 20:34

2 Answers 2


Dealing with mechanics

If you're going to take it to a mechanic anyway, you are probably better off letting that same mechanic do the diagnosis and obtaining the correct part(s). Bringing parts and a diagnosis to a mechanic is likely to get as warm a reception as bringing your own eggs and raw bacon to a restaurant and asking them to cook you breakfast.

Doing it yourself

However, replacing an O2 sensor isn't usually that difficult. The first thing to check for is actually leaks in the catalytic converter or the exhaust pipes and headers. A leak ahead of both O2 sensors will "look" to the computer like this problem and replacing the cat or sensors will just waste money on the wrong fix.

If you have access to an oscilloscope, you can look at the voltage levels on both the front sensor (pre-cat) and the rear sensor (post-cat). The front one should vary quite a bit and the rear one much less so. If you only have a voltmeter, you might be able to check the voltage while the engine is running. It should go from about 0.1V (lean) to about 0.9V (rich). You can usually force the mixture to go lean by removing a vacuum line.

You can also visually check the O2 sensors. If either one has got dirt or oil on it, it's probably due for replacement anyway. I've read some that say that they should be replaced every 100K miles anyway, but I'm not sure if that's really necessary on a well-maintained vehicle.

If you don't have a 'scope and you're willing to take a (possibly wrong) guess, I'd start with the post-cat O2 sensor.

  • Nice answer, including the bit about removing a vacuum line to force output lean. And I love the restaurant analogy!
    – zipzit
    May 11, 2016 at 18:24
  • Excellent summary. Aside from attempting anything myself, I'd just like to understand. Suppose the engine is at a steady 3000rpm, then the air-fuel ratio will remain at some constant value, and hence the upstream O2 sensor ought to give a constant reading on an oscilloscope, rather than some kind of wave. What am I missing? Also, 3000rpm is 50 Hz. Are we indeed talking about a wave at that frequency? I'd imagine the exhaust couldn't change O2 content that quickly.
    – Calaf
    May 11, 2016 at 21:39
  • This video should help a lot for understanding what the waveform looks like. It's quicker than you think, perhaps! youtube.com/watch?v=uYMGnvtgi8w
    – Edward
    May 11, 2016 at 22:03
  • Regarding exhaust leaks, even pinhole leaks in a convertor can lean out the rear 02 and cause a p0420. This may not be apparent unless the system is under pressure.
    – Ben
    May 11, 2016 at 23:29
  • @Ben Ah! Now I understand what the "pressure test" is about. Thank you. I presume that an exhaust leak is caused by either rust or impact. I posted a picture of my cat in a related question: mechanics.stackexchange.com/questions/29577/… This picture suggests that neither impact nor rust is the culprit, so it's unlikely that there is a leak. Is this about right or am I missing something?
    – Calaf
    May 12, 2016 at 0:31

Four things can throw this P0420 code,

  1. Bad catalytic converter

  2. bad upstream O2,

  3. bad downstream O2,

  4. fuel ratio problem, rich or lean, which can be even more complicated to sort out as suggested in the article.

  • Good point about the fuel ratio problem -- I neglected that possibility in my answer and it won't necessarily result in additional codes.
    – Edward
    May 11, 2016 at 18:41

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