Auto Service World
Feature   May 1, 2004   by Auto Service World

Knowledge Building: Gas Analysis and Diagnostics

Despite the fact that there are a number of jurisdictions in the country where emissions testing is mandatory, and that these programs have been top of mind for much of the industry for a decade, using emissions as a diagnostic tool remains elusive for many technicians.

Many technicians–your customers–continue to have difficulty determining what problems can cause which outcomes, which puts them in the unenviable position of effecting repairs that do not have the desired effect on emissions or driveability.

At the very least this causes a consumer to spend more money than necessary while that technician searches for the real cause; at worst it adds to the general bad opinion that many consumers have of the independent aftermarket.

It is generally assumed by much of the public that the advent of OBD II has meant that all the answers are laid out in front of the technician, a supposition that is patently untrue, but it is also true that analyzing the exhaust content of a vehicle that may or may not be registering a trouble code can be a very efficient way of determining if there is a problem. It can also help point a technician in the right direction.

The reason that so many technicians have trouble in doing this is that many were not taught the basics of combustion in the era of emissions. They should have committed these basic rules to memory, and so should you if you want to be able to have intelligent discussions with your trade customers.

There are five basic elements to concern yourself with when thinking about combustion by-products: carbon monoxide (CO); carbon dioxide (CO2); hydrocarbons (HC); and oxides of nitrogen (NOx). There are a variety of other by-products, such as sulphur compounds, but these relate more to the components of the fuel itself rather than the combustion.

Use of a four- or five-gas analyzer can help your customers troubleshoot both emissions and driveability problems. All five of these gases, especially O2 and CO2, are excellent troubleshooting tools. A judiciously employed gas analyzer will allow a technician to narrow down the potential cause of driveability and emissions concerns, helping him to focus his attention on the most likely cause, and all without requiring access to the OBD II information (though it is not a substitute).

Additionally, analyzing these five gases after a repair will help determine the effectiveness of the repair.

There are some general rules of emissions analysis:

If CO goes up, O2 goes down and vice versa. High CO readings indicate too much fuel (rich); high O2 readings indicate a lean condition.

If HC increases as a result of a lean misfire, O2 will also increase.

CO2 will decrease in any of the above cases because of an air/fuel imbalance or misfire.

An increase in CO does not necessarily mean there will be an increase in HC. Additional HC will only be created at the point where rich misfire begins (in the region of 3% to 4% CO).

High HC, low CO, and high O2 at the same time indicates a misfire due to a lean or EGR diluted mixture.

High HC, high CO and high O2 at the same time indicates a misfire due to an excessively rich mixture.

High HC, normal to marginally low CO, and high O2 indicates a misfire due to a mechanical engine problem or ignition misfire.

Normal to marginally high HC, normal to marginally low CO, and high O2 indicates a misfire due to false air or marginally lean mixture.

Sometimes false readings can cause a technician to go hunting for a problem that was never there. This can be due to a holed exhaust system drawing in outside air, or a secondary air injection system that was not disabled during analysis. A good check is that the combined CO and CO2 readings should always have a sum of greater than 6%.

It is also true that the catalytic converter can mask a problem, which is why you might want to advise customers to insert the test probe ahead of the catalytic converter.

Understanding NOx

NOx gases are often looked at less than the other four components of combustion, but should not be ignored.

NOx are created when combustion temperatures are abnormally high, allowing the atmospheric nitrogen and oxygen to combine in the cylinder.

Common causes of excessive NOx content in the exhaust gas are cooling system problems, such as insufficient radiator airflow, low coolant level or poor cooling fan operation, a stuck thermostat, as well as fuel system problems such as an excessively lean mixture, or leaks in the intake manifold. A lazy O2 sensor or one that is biased to rich voltage can also cause high NOx readings. EGR problems, whether due to a malfunctioning valve or blocked passages, can also cause NOx to rise.

Improper spark advance, knock sensor problems (which cause the first problem), and carbon build-up that can raise the compression ratio are other causes.

Here are some real world emissions readings and the limits as determined by Ontario’s Drive Clean program. What do you think the problem might be?


HC ppm1353


NOx ppm15532013

HC is well within the limits, and CO is perfect. The NOx reading is high, however. Accordingly, these would seem to indicate high combustion temperatures, but complete combustion nonetheless. This vehicle is not equipped with an EGR valve; it is a 1986 model VW, so we can eliminate EGR problems as a cause.

Looking at the list of possible causes, it could be due to spark advance, a thermostat in need of replacement, or poor cooling. So why did the technician clean the throttle body and replace the spark plugs (platinum plugs which only had about 25,000 km on them)?

The car did pass a subsequent test with an NOx rating of 881 ppm–low enough to pass, but higher than it should be–but this could have been due to the engine not being as warm for the retest.

If the technician had truly understood the cause of these readings and looked for an obvious cause, he might have taken a look at the coolant reservoir and noticed that the coolant level was quite low, which is the most likely cause of the problem.

This is just one example of misdiagnosis that can result from a lack of understanding about what the by-products of combustion can tell you.

On the flip side, intelligent use of the information can provide a quicker method to diagnosing a problem than trouble codes alone. Exhaust gas analysis can speed up diagnosis, and provide more information because it is a dynamic test conducted while the vehicle is running.

Considering the difficulty that many independent garages continue to have in billing for diagnostic time, it may be an approach you should suggest–though you might want to accompany it with the suggestion that they receive the appropriate training, lest they find themselves looking in the wrong place for problems despite the added information.