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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?

Calaf
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2 Answers2

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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.

Edward
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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.

Moab
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