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Diagnosing Catalytic Converters

GhostWriter

By GhostWriter
in Ask A GM Technician

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GhostWriter Enthusiast

GhostWriter

Posted
Posted

Recently, a local mechanic advised my girlfriend that she "probably" has a clogged catalytic converter on her 2007 Silverado.

 

I can think of a few ways to help determine this, but was wondering what protocol, if any, a GM technician is required to follow.

 

For example, would they be required to place a pressure gage in the downstream O2 port?

 

Thanks for your help! 

davester Senior Enthusiast

davester

Posted
Posted

They measure the pressure at the upstream O2 ports (between the engine and the cat's).  IIRC, if it's more than about 2 psi (the number around there), then the cat should be replaced.  This is from my '04 Sierra's full-service manual.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

I would expect you would need to measure the pressure both before and after the catalytic converters.  If you only measure before the cats you may end up replacing a perfectly fine converter when the restriction is actually a mouse nest in the muffler.  This brings up the question of did the girlfriend complain of no power under hard acceleration, or did the shop come up with this diagnosis on their own? 

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Normally in the interest of time, we just remove both upstream o2s, or, if the vehicle is light on rust, disconnect the Y-pipe, and see if the truck runs better. Pressure testing after that if the results are inconclusive.

 

If the truck has been run with a dead miss, or a severe misfire for a while, you can be pretty sure the cat is toast.

GhostWriter Enthusiast

GhostWriter

Posted
  • Author
Posted

Thanks for the insight, guys.

 

To be more specific, she complained about being a little down on power.  Most noticeably when climbing some of the steeper hills in the area.

 

Apparently the mechanic, a family friend, performed a quick inspection using an infrared thermometer and determined one of the cats wasn't operating within the correct temperature range.

 

I do most of my own wrench work, but have never had a converter fail.  Didn't know how to advise her.

 

Wasn't sure how reliable the operating temperature test would be.

 

Again, thanks for your help!

 

 

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

You may want to make sure the fuel filter is not plugged as well.  Fuel injected engines behave like they have restricted exhaust/plugged converter when the fuel filter gets plugged.  Converters are designed to operate in a certain temperature range, and that will vary with the load on the engine and the amount of fuel being used. I think there are too many variables involved to trust the temperature reading as a diagnosis. 

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Yep - forgot about that. That's another quick an easy test - even easier and quicker than removing an o2.

 

A good cat should always be around 100° hotter at the outlet than the inlet. Hotter or cooler than that, there could be an issue, depending on how much.

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