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Common Emission Problems

Today, cars that fail emissions because of high NOx readings are extremely common. NOx, or NO in some jurisdictions, stands for oxides of nitrogen. NOx should not be confused with Nitrous Oxide NOS, which is a very different substance. To know how to diagnose NOx problems, you first have to understand what causes NOx. There’s that “how stuff works” issue again.

Nitrogen from the air becomes NOx when fuel and air burn at high temperature, i.e., when combustion temperatures exceed 2,200 degrees Fahrenheit. At this temperature, nitrogen atoms are broken apart and combine with oxygen to form a new substance, NO2—Nitrogen dioxide or NOx.

Now that we know NOx emissions are created through high combustion temperature, we next have to understand what things can cause the high temperature. In modern engines, it is normal to have very high combustion temperatures. So in most engines, it is necessary to cool the burning process.

The most common way to do this is Exhaust Gas Recirculation (EGR), which recirculates cool exhaust gas back into the cylinders to lower combustion temperatures. Cooling the combustion process is the only job of EGR. So any failure, even a small failure in the EGR system will elevate NOx.

Next on the list for high NOx is a lean fuel mixture. As fuel mixture becomes lean combustion, temperature climbs, and engine efficiency drops. Contrary to popular perception, when you cut back on fuel, the mileage drops, but NOx climbs. A too-lean fuel mixture can be caused by low fuel pressure, low fuel delivery volume, dirty fuel injectors, air or coolant temperature sensors that indicate a higher than actual temperature, and the No. 1 lean mixture culprit—an out of spec MAF sensor.

Diagnosing NOx failures is mostly pretty simple. If the car has EGR, begin by checking the entire EGR system and not just the EGR valve. EGR valves fail, sometimes. EGR passages become restricted, always. Every mile means a tiny bit more deposit in the EGR passages. Because clogged passages are an absolute, it’s beneficial to include regular EGR system service into your PM schedule.

There are various processes on the market, but a dedicated system such as the one made by BG Products will be your best choice. By servicing your fleet’s EGR systems every 30,000 to 50,000 miles, you will prevent the majority of NOx failures on your EGR equipped vehicles.

If the failed vehicle doesn’t have EGR, or the EGR system passes basic tests, look at the inlet air temperature and coolant temperature sensor readings next. This can be done with your scan tool in the DataStream portion of the test procedure. Compare inlet air temperature readings to ambient temperature in front of the vehicle. If the temperature reading is notably higher than true ambient temperature, the sensor is bad. Unfortunately, many inlet air temperature sensors are built into the MAF and can’t be replaced without replacing the entire MAF sensor.

Coolant temperature readings are similar, except the measurement is taken at the thermostat housing. The actual temperature then is compared to the scanner display. Again, if the scanner temperature reading is significantly higher than true temperature, the mixture will be lean, and NOx will climb. I frequently find coolant temp sensors indicating 300 degrees or higher when the engine is actually running at about 200 degrees. Temperature readings should be taken with an infrared thermometer, and the temperature at the outside of the thermostat housing will be about 15 degrees lower than actual.

Last but by no means least is the MAF sensor. This is second only to EGR for creating problems. And don’t expect a bad MAF to automatically set a code because it probably won’t. It is normal for MAF sensors to move toward lean as they age.

Although MAF sensors work in a number of ways, the underlying principle is generally the same. Inside most MAF sensors is either a piece of wire or conductive film. In either case, electricity is passed through a conductor causing it to heat up. As air passes over the heated element, there is a temperature change, which in turn is detected by the computer. More air equals a greater temperature change. As the readings to the computer change from more or less air movement over the element, the computer matches the air with the proper amount of fuel. More air, more fuel; less air, less fuel.

But why do MAF sensors change values as they age, and why is that change almost always toward lean? Simple. Over time, dirt collects on the heated element, which dampens its heat reaction to air movement. In other words, it becomes anesthetized to air movement and underreports the amount of air entering the engine. Because the computer thinks there is less air than there really is, it delivers too little fuel for the true amount of air, leading to a lean fuel mixture.

More dirt equals less reaction, less reaction equals a more lean fuel mixture, a more lean fuel mixture means higher combustion temperature, and that, folks, creates higher NOx. And because it often isn’t possible to clean a MAF sensor, replacement may be necessary. Furthermore, because the sensor still is within range, just not accurate, there often will be no codes.

When all other reasons for high NOx have been eliminated, replace the MAF. These are not the only causes of high NOx, but they are the most common. Also, be aware that out-of-spec MAF sensors cause many no-code performance problems.

Pat Goss is the resident master technician for Motorweek TV (PBS), a columnist for the Washington Post, and the host of radio and cable shows discussing vehicle maintenance. He also was a consultant to the Prince George’s County, MD Police Department on fleet maintenance, and he is the president of Goss’ Garage in Seabrook, MD. He can be reached at

Published in Police Fleet Manager, Jul/Aug 2006

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