Catalytic Converter testin usin live data question

[Steve Webb]

Hi Eric, how are you?

I have a code reader / scanner which reads live data and it is pretty much a carbon copy of your Actron one.

I’m about to test my cat. My question is this:

In the video on your Subaru you have two O2 sensors, one pre-cat and one post-cat. I get that you need to see the first O2 sensor switching rapidly and the second one just sits there at around half a volt.

Well, i have a V6 with two exhaust manifolds. i also only have one O2 sensor in each pipe pre-cat. so two in total. What should i be looking for on the live data?

Also, this test will confirm if the cats performing but will it also confirm a blockage ?

Topic

[EricTheCarGuy]

In your case I would use the temperature test. That way you know for sure which catalytic converter is bad if indeed that is the case.

BTW there should be a sensor either inside the cat or after it if it’s capable for monitoring catalytic converter efficiency.

[Steve Webb]

Thanks Eric.

I’ll do that. Will just that test alone confirm if it’s ok and if there’s a blockage?

I’m presuming then that my car doesn’t have a efficiency monitor sensor inside (and definitely not one after it) as no one has ever mentioned such an item on my forum.

[Its me]

You can determine blockage with a either a backpressure tester or the simpler way with a vacuum gauge… Either test will let you know something is amiss…

A code (I.E. P0420 or P0430) in the simplest sense just means that the downstream O2 sensor is reporting unusual data…

-Karl

[Steve Webb]

Thanks TK,

How would I use a vacuum gauge to do that? Thanks.

[Its me]

Here ya go…

[Steve Webb]

Thanks very much for that!

Two Q’s after watching that video…

Am I looking to compare my figures to a manufacturers spec or am I looking simply for vacuum to drop off with 2k RPM applied?

And why does this happen? I can’t see how exhaust would affect vacuum (obviously it does but just would like to know how / why) 🙂

Thanks

[Its me]

If you can find out from your manufacturer what your car should be, that can help…. Typically between 18-22 inches of mercury (in-HG) is where you want to be… However elevation can effect this vacuum… So a car in the flatlands of Kansas should be around 18-22 but if you lived in Denver, it may be much lower (see readings below)

Sea level-1000 ft. 18-22

1000-2000 ft. 17-21
2000-3000 ft. 16-20
3000-4000 ft. 15-19
4000-5000 ft. 14-18
5000-6000 ft. 13-17

What you really are testing to see if you have a blockage is what happens to the needle (I.E. it drops)

As Eric said, in the old days, the vacuum gauge told a story if you knew how to read one, but is not as commonly used anymore…

Here is a good link with a chart for diagnosing other things with a vacuum gauge…

http://www.gregsengine.com/using-a-vacuum-gauge.html

-Karl

[Steve Webb]

Karl, thanks so much for your input. That’s really useful info! 🙂

[Author]

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