Catalytic Converters

By Brandon Baldwin

              A catalytic converter is an emissions control device on our vehicles that takes leftover hydrocarbons (uncombined fuel) and leftover oxygen in the exhaust and combines them to into carbon dioxide and water vapor.  Now, perfect combustion is supposed to take place  within  the  engine  to  produce  the  byproducts of only carbon dioxide and water vapor.  But, being that an efficient engine is still only 30% efficient, we still have unburned fuel that exits the combustion chamber.  (By now, you might be asking, “where does the other 70% of the fuel’s energy go?”  Right?  It goes to heat.)  The catalytic converter is by far the most effective device for reducing the emissions of an engine.  These byproducts that harm the environment are hydrocarbons HC, carbon monoxide CO, and oxides of nitrogen NOx.  In other words, it adds oxygen to the hydrocarbons, oxygen to the carbon monoxide, and reduces the oxygen from the oxides of nitrogen.  The oxygen comes from the leftover oxygen in the exhaust stream that didn’t get burned/combined with the hydrocarbons (fuel).  

P0420 and P0430 are some of the codes that get thrown around instead of saying “a failed catalytic converter”.  When you have one of these codes, this means that the oxygen sensor that is placed after the catalytic converter (cat for short) has detected a variable oxygen level similar to the oxygen sensor before the cat.  When that happens, the Engine Control Module interprets this as a failing cat.  The check engine light comes on because this means the emissions have exceeded 1.5 times the Federal Test Procedure: emissions are too high.  

Ways you can check your cat include scoping the downstream (after cat) oxygen sensor and temperature checking.  In both cases, you need to run the engine above 1000 rpm and hold it there at least 5 minutes before testing.  This is to make sure that if the cat simply needs to “light-off” it has time.  It will get hot, I mean over 500 degrees hot!  But, the cat is more efficient at this rpm (that’s what you would normally drive around at), and that temperature makes the oxygen sensors work more efficiently too.  They cycle around 1x per second.  (Air Fuel Ratio AFR’s are a little different.) Then, if you are scoping, you should see a nearly level line, not varying more than 100mv on the post-cat oxygen sensor.  If you are looking at temperature, then you should see about a 10% increase in temperature after the cat.  I do both because I want to be sure.  Catalytic converters aren’t cheap, especially California emissions ones.  So, it best to check more than one way to be absolutely certain.  

If you determine that you need to replace the catalytic converter, you need to check your emissions sticker located under the hood to see if indeed you need a California Emission TWC (three way catalyst).  If your car was sold in New York State, you can almost guarantee that it will be a California Emissions car.  Some trucks follow different emissions rules.  The dealership would be the logical choice to purchase a new one and have it installed.  But, you can buy aftermarket ones that can outperform the original and still be completely emissions legal.  Yes, in the “old days” catalytic converts restricted exhaust flow which reduced performance.  These were the pellet style catalysts.  But, now there are high flow cats on cars.  There are even higher flow cats from companies such as Magnaflow.  Personally, I have had to install catalytic converters on many cars and have been very satisfied by the Magnaflow converters.  I’ve even seen performance improvement.  

You probably thought this was done.  Not so fast buddy!  Catalytic converters don’t just normally fail.  Something made them fail.  Of course, physical impact, such as on an off-road vehicle will cause failure.  They can break.  But, for most of us who travel on the road have had another cause.  If you ignore the check engine light for a long time, you are asking the cat to clean up the poor running mess in the exhaust.  A misfire is a great example of this.  One spark plug isn’t firing, an exhaust leak before the first oxygen sensor, a vacuum leak at the intake manifold, a leaky injector, a stuck open thermostat, and so on.  These can all lead to catalytic converter failure.  Of course, it’s a waste of gas too.  So, if you get a P0420, fix the problem that created it, or you will be installing another converter in the future too.  Yes, there is paperwork for that.

Happy Engining.  

Return to Tool Box