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Emissions Testing

If you live in an area that performs emissions testing, you know that it can be frustrating for the vehicle owner when they fail an emissions test since this typically means that they can not renew their license plates without either getting it repaired or spending a minimum fee in an attempt to repair the issue.

There are two types of emissions tests commonly used by various EPA programs. The first is a 5-gas emissions test while the second relies on the fact that OBD 2, with its many monitors, effectively tests the various systems on the vehicle and will illuminate a check engine light if it detects an issue. In many ways, technicians find the later way of testing easier to repair once the system has failed. In this article, we will address the 5-gas test method since this is typically more difficult for technicians to understand.

EMS 5-Gas Analyzer with Lambda Readings

The gas analyzer uses lights and mirrors to sample the incoming exhaust.

Each gas sampled will allow certain shades of light to emit through it.

The analyzer will use the percentages to determine the amounts of each of the gasses we discussed.

The 5 Gasses tested during an emissions test:

The 4-Stroke Cycle - What is happening?

HC = HydroCarbons

Hydrocarbons (HC)

These are unburned gasses and are an indicator of an issue. What is the issue? We don’t know without looking at the other gasses. Some possibilities are burning oil, lack of compression, over advanced timing, too much burning of fuel or too lean (not enough fuel) In other words, anything that causes combustion to have an issue.

02 = Oxygen

Oxygen (O2)

This is our lean indicator. Although we can’t fail an emissions test for too much oxygen, it is a great indicator of a lean condition such as a misfire.The air we breathe is made up of 21 % oxygen. If a misfire occurred, we would not use the oxygen during the act of compression and thus we would have a high level of oxygen present and since that is an issue, HC would also be high!

c02 = Carbon Dioxide

Carbon Dioxide (cO2)

This is our efficiency indicator. A normal gasoline engine is considered to be running very efficiently at 15%. So if we have any issue what do ever, the efficiency would drop. In the above examples of running lean due to a vacuum leak or ignition misfire, the HC would be high, the 92 would be high and the cO2 would be low.

cO = Carbon Monoxide

Carbon Monoxide (cO) is our rich indicator.This can be caused by anything that causes too much fuel to be burned! If we are burning too much fuel it will cause the efficiency to drop as well as the HC and cO to be high with low 02! It is important to note that in order to have c0 or c02 or n02 one will need to have combustion occur. Hopefully we start to see how all the gasses are relating to one another.

NOx = Oxides of Nitrogen

Oxides of Nitrogen (NOx) is our heat and load indicator. N0x is created by danced timing, high combustion chamber temperatures. In order to reduce this gas one must lower the temperature such as richening the mixture, or having variable valve timing or an eve valve to allow extra air into the cylinders which will reduce combustion chamber temperatures.

In the early days of computer controlled vehicles, the highest limits for HC

was 220 parts per million, cO was 1.2% and NOx was roughly 1500 parts per million. Newer vehicles tightened the standards some due to more efficient running engines. Notice there is no limit for 02 or c02 since those are not considered poisonous gasses.

The most important thing to know when diagnosing vehicles with failed emission gasses is to understand how to interpret their meanings and how they all relate to one another. Now that you have an understanding of the gasses and how they relate to one another, diagnosing the reason for failed emissions should be easier.

Potential issues for different types of issues:

Now that we understand how these 5 gasses play with one another, let’s describe a three-way catalytic converter’s operation since it has a main purpose of helping to clean up these gasses.We should note that the converter needs to have no exhaust leaks present and also needs some heat, about 600 degrees to function correctly.

Computer controls are designed to switch between a rich and lean mixture consistently every 100 ms. Or less. This is intended to allow the three precious metals inside a converter to work properly. Platinum and palladium are two of these metals designed to clean up HC and CO. RHODIUM is the third metal whose purpose is to clean up NOx.

When a vehicle is running lean, it will produce an excess amount of oxygen.The three-way cat will store this oxygen in its honeycomb substrate to enable releasing these excess oxygen molecules when they need to clean up HC OR CO. If we added 2 02 molecules to HC we create H20 or water. This is why it is common to see water dripping from a tailpipe of a vehicle.The converter is cleaning up HC!If we add two 02 molecules to CO we create C02 or carbon dioxide.Again this is considered a non-poisonous gas.

The mixture must be driven rich to help clean up NO2 or nitrogen. This is the reason why the computer must be constantly switching the fuel mixture between rich and lean to enable the proper function of the 3-way catalytic converter.Due to these three precious metals found within a catalytic converter, there is a high theft rate of these components amongst thieves so they can be scrapped and their precious metals can be sold.

What is Stoichometric?

Stoichiometric fuel economy is often referred to as 14.7 to 1. This is 14.7 parts air to every 1 part of fuel. This was a result of the above graphic showing that this ratio is a happy mix of the 5-gasses where all of them at this level would be a happy compromise of being at their lowest readings.

For many years, this was the targeted ratio, however, we should note that as technology has improved, we are able to run vehicles leaner than we were able to in the past!


So what if we do not have a 5-gas analyzer? Rest assured that in most cases the OEM's will display a lambda reading on the Global side of OBD 2 Diagnostics.The issue with using a 5-gas analyzer as an emissions testing tool (there are many other diagnostic tests that can be performed with an analyzer), is that the modern-day converter has the ability to mask various engine running issues. Since Lambda is not affected by the catalytic converter, it is a better way to see if the engine is suffering from rich or lean operating conditions.

Now that we have a better understanding of the 5-gas analysis and how computer controls and catalytic converters work, it should make diagnosing issues resulting from a failed emissions test a great deal easier.

By: Electron-John

Technical Trainer Specialist for Autel

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