Our job as automotive technicians is to repair vehicles as quickly and accurately as possible. There isn’t a set way of diagnosing a vehicle as long as you repair the vehicle and the problem that it came in for. Working on today’s high-tech vehicles is like going to war. While at war you want to slay your opponent by any means possible before they slay you. So choosing the most efficient and accurate weapon will go a long way in winning the battle. The same goes for diagnosing vehicles that have any type of drivability concerns. The most common weapon of choice used on the modern automobile is the scan tool. That tool will give you the most information in the least amount of time. Along with the scan tool, there are various other pieces that diagnostic equipment that will aid in your diagnosis... for example, the thermal imager, the infrared thermometer, the pulse sensor, and even the pressure transducer. Also along with those add-ons, a digital volt-ohm meter and lab scope are sometimes necessary for completing an accurate diagnosis.
In addition, there are more tools that you can use to fight your diagnostic battle. One tool that has been around that many technicians haven’t thought about using is the five gas analyzer. The first thought that comes to mind from many technicians is the fact that they think a five gas analyzer is only used for emission testing. There are a lot of states that don’t perform emission testing, so the chances are those shops don’t use a five gas analyzer. Using a five gas analyzer for just emission testing is only one part of the equation, and a small part at that. Understanding how a five gas analyzer works and utilizing it to its fullest potential can be just as valuable as a scan tool. The gas analyzer has been around for many years. In the beginning, it was a stationary tool that was pretty large and cumbersome to move around the shop (kind of like the old big-box oscilloscopes). Fast forward to the present time and now gas analyzers are available that can fit in the palm of your hand. They’re more versatile to use and can even be taken on a road test. (Fig 1)
Just like new technology in the automobile that has evolved over the years, five gas analyzers have evolved as well. In this article, I’m going to show you what a five gas analyzer can do to help you in your diagnostics. Remember, this is just another tool that you can use to safely and accurately diagnose vehicles. So let’s take a look at what the five-gas analyzer really is. The five gas analyzer is a device that measures the content of the exhaust byproduct after a combustion event. This is done by inserting a probe or more commonly known as a detector into the vehicle’s tailpipe. Once the detector is installed there are sensors built into the probe that are used to measure different gases in the exhaust mixture. Infrared light is directed through the sample gas in the sample chamber and the amount of light passing through the sample is measured by the detector. There are normally three detectors used for a five gas measurement. The exhaust gas stream goes into the inlet part of the detector and then an infrared light beam is shot through the gas sample which is bounced off of an optic filter. Then the gas sample will exit the detector. The three gases that the detector focuses on are carbon monoxide, carbon dioxide and hydrocarbons. (Fig 2) Nitrous Oxide or NOx and Oxygen O2 are also tested as well. The deadliest or most harmful of the five gases is Carbon Monoxide or CO. A major component of polluted air, CO is a colorless, odorless gas that, when concentrated, is toxic. Any fueled engine properly adjusted and in good operating condition produces relatively low CO readings. Excessive CO develops when the combustion chamber is receiving a large volume of fuel and a proportionately small amount of air. By weight, carbon monoxide accounts for about 47% of air pollution produced by vehicle emissions. Think about a cylinder misfiring that has lost its spark, and all that extra unburned fuel exiting out the cylinder into the catalytic convertor then out the tailpipe. Does a flashing check engine light signal a catalytic convertor damage warning come to mind? The CO reading on your analyzer will be extremely high. (Fig 3) Normal CO readings will be 0.5% or less. A five gas analyzer will catch a CO problem before it gets too bad. It may or may not see the problem before your scan tool will. This is just one example of the benefits a five gas analyzer will have in your diagnostic testing.
But why test the engine for how clean it’s running? Have you ever seen pictures of the skyline over Beijing China? That haze you saw is smog. There was a great concern about having the Olympics there because of that. So what actually is smog? Smog was found to be the result of a chemical reaction that occurs in sunlight between hydrocarbons (HC) and oxides of nitrogen (NOx ).
Once the government understood the contribution of vehicle emissions to air pollution, it formulated legislation to control it and that’s why we have strict vehicle emissions standards today.
Let’s take a look at the comparisons of a rich and lean running engine from a five gas analyzer and compare it to the scan tool readings. Notice the CO reading on the analyzer is showing an abnormally high reading. (again, see Fig 3) The system is showing a rich condition. The Oxygen sensor reading from the scan tool is showing a high voltage reading indicating a rich condition. This was done by injecting a propane sample into the intake system to see the reaction of both the five gas analyzer and the scan tool. The scan tool picked up the rich fuel sample before the five gas did due to the location of the oxygen sensors located upstream from the five gases probe that is located in the tailpipe. It usually takes a few seconds for the enriched sample to reach the five gas probe. This is normal, due to the distance the sample has to travel. Notice the Lambda reading on the analyzer. It is showing a reading of 903. Lambda is an indicator of the air-fuel mixture in the engine. A perfect Lambda reading is at one. Anything less than one indicates there is more fuel in the mixture than there is oxygen. Anything higher than one indicates there is more air than fuel in the mixture. Notice the high CO reading and the lower Lambda reading. That’s a sure indication of a rich running condition. Figure four is showing us the opposite. (Fig 4) The engine is now showing a lean condition which was induced by removing a manifold vacuum line at the intake manifold. Notice our CO has dropped significantly and our Lambda is now above one. The vehicle’s oxygen sensor is now reading a low voltage as well. The vehicle’s oxygen sensors are also proving that the computer system is controlling the fuel mixture as well. Notice in both different scenarios the CO2 or carbon dioxide is a little low. The spec for CO2 is 14-15%. Anything less than that is telling us the engine is showing us that the engine is not running as efficiently as it is supposed to be. This is where a five gas analyzer will shine and a scan tool will not. The scan tool did not show any codes for these two different gas readings. My guess over time once the vehicle’s fuel trims started to change drastically they would have. The point I’m trying to make is the gas analyzer will pick up the overall efficiency of the engine after the combustion event where the vehicle’s computer might not catch the problem as it’s happening. This could be due to a biased sensor, an intermittently reading sensor or a computer software problem. Remember, I want to stress that the five gas analyzer is in no means used to replace the scan tool but another way of diagnosing drivability problems. Each tool complements the other. The Hydro Carbon or HC is showing that it was reading high in our rich running reading. That is telling us that the cylinder(s) are not generating complete combustion of the gases. HC is measured in PPM or parts per million. A normal reading is 100ppm or less. Anything over that is indicating a cylinder is experiencing a lack of ignition and excessive unburned fuel. Our reading of 180ppm was showing us just that. What makes the use of using a five gas analyzer is taking all the readings and comparing them to what is normal and what is not. You can see by doing that you will be able to determine what is wrong with the engine and how far off that engine actually is compared to a known good reading. I tell all my technician friends and students that whatever tool you are using, the best way to understand them is to test a vehicle that is known well. If you don’t know what known good is how are you going to know what known bad is telling you?
The five gas analyzer can be used not only test at the tailpipe but it can be used for other tests as well. For example, we know that when an engine’s head gasket might have failed or is about to fail that the engine will induce CO2 into the coolant system. Sure, we have block testers that we can use or balloon tests on the coolant reservoirs but how about using the exhaust probe of our five gas analyzer to test for CO2 in the cooling system?
The best way to do this is to remove the radiator cap or coolant reservoir cap. Do not insert the probes end directly into the coolant, as that is a sure-fire way to need a new replacement tip for your gas analyzer. Make sure the coolant level is lower than the top of the tank you are testing as if it’s a big leak you don’t want the coolant to come into contact with your probe as the engine starts to warm up. Hold the probe above the coolant level and watch the analyzer, specifically your CO2 reading. If the CO2 reading is high then more than likely your head gasket has failed. Other uses for the five gas analyzer would be to check the exhaust system for leaks by running the probe around all the connections and along the pipes for any sign of abnormal CO2 readings. Using the analyzer to check for fuel leaks is a great way of finding those pesky fuel leaks that sometimes you can’t see but can smell. Another great use of the gas analyzer is to check the integrity of the catalytic convertor. If exhaust emission test results exceed manufacturer specifications and HC cannot be adjusted to within limits, the problem could be an inoperative catalytic converter
Ensure the vehicle has no ignition problems, vacuum leaks or fuel restrictions. On vehicles with feedback control, disconnect the O2 sensor to prevent it from correcting during this test. Disable the air injection system. Richen fuel mixture until CO is about 2% using propane. With mixture still enriched, enable the air injection system. Note HC, CO and O2 readings. O2 increases and HC and CO decrease significantly when the air injection system is enabled when the catalytic converter is working. O2 increases and CO and HC readings do not change significantly when the air injection system is enabled when the catalytic converter is not working. If O2 is higher than CO, and CO is higher than 0.5%, the catalytic converter is not functioning (not converting CO to CO2). If O2 is lower than CO, the exhaust sample is rich and the converter is oxygen-starved, not oxidizing HC and CO. After correcting a rich condition the converter should begin to function again. It may take several miles of normal driving, or an engine operating at a fast idle speed for about 15 minutes before this happens. Remember, catalytic convertors are expensive and keeping the vehicle maintained is the best practice in preventing a catalytic convertor failure. (Fig 5) Finally, another use of the gas analyzer is the separate testing of NOx on a diesel engine. With today’s after-treatment systems, testing the diesel particulate filter for efficiency based on the SCR catalyst’s performance is vital in diesel emissions testing. I will talk more on that in a future article.
As you can see, using a five gas analyzer is not just for emission standards testing but it can open up a whole new avenue for your ability to perform drivability diagnostics. When diagnosing a vehicle it is important to use all the resources you can obtain. This article has only touched on a small portion of what a five gas analyzer can do. Efficiency is key to an accurate diagnosis. With the proper training and the use of quality tools then you too will have success repairing vehicles in your bay. Using a five gas analyzer in your diagnosis can be…well a gas!
