A look at diesel SCR service

Nov. 2, 2020
It’s just good business to be prepared to work on these vehicles and to educate your customers about diesel engine ownership.

The use of diesel power for cars and light trucks in the U.S. is on the rise. As more Americans drive larger SUVs and pickup trucks the demand for diesel engines has increased due to their reliability, fuel efficiency, and lower cost of operation. Sales reached an annual level of three percent of total vehicles, higher than hybrid and electrical automobiles. The largest group of vehicles that use diesel engines are pickup trucks like Ford’s F150 (class 2) and F250 (class 3) Chevy and Dodge 1500/2500 series. Over the last three years, buyers of light trucks have checked the diesel engine option enough to grow diesel usage 35 percent. Even diesel sales for cars and SUVs have shown a growth of 2.4 percent in 2018 (increasing to 7.6 percent in 2020). If your shop doesn’t currently perform much repair work on diesel vehicles, keep this in-mind. The chances of one showing up for service are increasing. It’s just good business to be prepared to work on these vehicles and to educate your customers about diesel engine ownership.

Figure 1 - This 2010 BMW 5 Series 3.0L four-cylinder, twin-turbo, diesel engine is required to have a  selective catalytic reduction (SCR) system to control NOx emissions. The 3.0L outputs 425 lb-ft. of torque and 265 hp plus returns EPA mileage of 19 mpg city and 26 highway thanks in part to SCR technology.

Diesel engines are more fuel-efficient than gasoline engines because diesel fuel contains about 10 percent to 15 percent more heat energy. This energy advantage translates to 20 percent to 30 percent more miles per gallon than a similar-sized vehicle powered by gasoline. Unlike conventional gasoline engines, where power is controlled by the air/fuel mixture, diesel engine power is directly controlled by the fuel supply. This creates very lean mixtures of 25:1 (or higher), even at full power. The lean-burning nature of diesel engines comes at a price because lean mixtures produce high combustion temperatures. This results in significant production of nitrogen oxides (or NOx). With no emission controls, diesel engines produce 20 times more NOx than a gasoline engine of the same size.

Similar to gasoline engines, diesel engines use exhaust gas recirculation (EGR) to reduce NOx. EGR works by recirculating a portion of the exhaust gas into combustion chambers. This inert gas dilutes the oxygen in the incoming air stream and absorbs combustion heat reducing peak temperatures. Lower combustion chamber temperatures facilitated by EGR reduces the amount of NOx in the exhaust.

The downside of EGR diesel applications is that engine performance and fuel efficiency are decreased. Also, EGR gases cause engine oil to become contaminated rapidly resulting in more frequent oil changes. Because EGR reduces the amount of fuel burned during the power stroke, particulate matter (PM) or soot is increased. The very design of a diesel engine causes a conflict in chemical factors between PM and NOx. When the engine is operating most efficiently for power, minimal PM is produced but NOx levels are high. When exhaust gas is recirculated back into the intake manifold through the EGR system NOx is reduced but the PM levels are high.

Diesel exhaust soot is reduced by the use of a diesel particulate filter (or DPF) that captures and intermittently burns off about 90 percent of PM. The very nature of the operating principals of a diesel engine makes the removal of NOx a challenge as EGR alone does not reduce it enough to meet 2010 and later emission standards. The only method to remove NOx gases from diesel exhaust (without effecting engine performance) is to treat the exhaust.

NOx reduction via SCR

Selective catalytic reduction (SCR) is an exhaust system emissions control technology that converts NOx into nitrogen, water, and carbon dioxide—all components of the air we breathe (figure 2). SCR technology reduces NOx by 90 percent through the use of a catalytic converter, where the NOx is oxidized and converted into other elements. It is called “selective” because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The word “reductant” is a scientific term used to state that a compound (NOx in this case) chemically changes into other compounds like nitrogen and water inside an SCR catalyst.

Figure 2 - Diesel engines are fuel-efficient because they operate on very lean mixtures that produce high levels of NOx. This output is reduced when NOx molecules in the exhaust are mixed with ammonia from the diesel emission fluid (DEF) tank, and converted into nitrogen and water, in the SCR catalyst.

SCR technology has been used for decades in stationary diesel generator sets; marine vessels like cargo ships, ferries, and tugboats. SCR for NOx reduction was used for coal-fired generation of electricity as early as 1975 in Japan and 1994 in the U.S. SCR has been applied to heavy-duty diesel engines in the United States and Europe since 2007. In 2010 the Environmental Protection Agency (EPA) required the reduction of NOx in diesel-powered light trucks and automobiles, to the extent that only vehicles equipped with SCR technology could pass the new emission standards.

Figure 3 - All DEF containers should have this American Petroleum Institute (API) label that certifies the quality of the DEF fluid. DEF can be purchased at auto parts stores, Walmart, truck stops, and some convenience stores.

To reduce NOx the SCR system injects ammonia into the engine’s exhaust. Diesel exhaust fluid (DEF) provides ammonia used in the SCR process. DEF is made up of urea and deionized water. Urea is a waste product found in the urine of animals. Automotive-grade urea is synthetically produced and has been in use as an inexpensive form of nitrogen fertilizer for many years. Deionized water has most minerals removed such as sodium, calcium, iron, copper, and other elements. The ratio of urea to water in DEF is 32.5 percent urea and 67.5 percent deionized water. DEF is available at auto parts stores and truck stops and comes in a variety of containers including bulk, bottles or jugs. The American Petroleum Institute (API) rigorously tests DEF to ensure that it meets industry-wide quality standards and their certification label appears on all approved DEF containers.

In addition to reducing NOx emissions, SCR simultaneously reduces HC and CO by 50 to 90 percent, and PM emissions by 30 to 50 percent. In some applications where SCR and PM filters are combined, fuel efficiency has increased from three to five percent. While SCR has a positive effect on exhaust from diesel engines and increased fuel economy, it does have some downsides. SCR system components including the DEF tank, tank heater, DEF injector, level sensor, temp sensors, NOx sensors, and the SCR catalyst all add cost to the vehicle. And like any other system, SCR components have to be maintained and/or repaired.

SCR in action

Figure 4 shows a generic SCR system and how the ECU controls its operation. Not all the components in this graphic are found on all vehicles. Starting with the DEF tank, the fluid level sensor inside the tank provides information to the ECU, or dashboard DEF gauge (if equipped) regarding how much DEF is available.

Figure 4 - Upstream NOx sensor data is used by the ECU to determine how much DEF to inject into the exhaust. The downstream NOx sensor checks that NOx levels have been reduced after the SCR catalyst. The temp sensor in the DEF tank provides the ECU with information about DEF aging and when to turn on the tank heater.

The tank temperature sensor input signals the ECU to turn on the heater, which keeps DEF fluid from freezing. The heating element can be electrical or engine-coolant based. Another function of the DEF tank temperature sensor is to help the ECU determine the service life of DEF. When in-tank temperatures are above 104º F, DEF ages/expires in only two months.

Figure 5 - This Fluid-o-Tech gear pump provides a precise, high-pressure source of urea for diesel SCR systems in passenger cars and light trucks. The Italian company is a major, global OEM supplier for automotive emission control equipment.

The pump or doser valve provides pressurized fluid to the DEF injector that sprays DEF into the exhaust (Figure 5). On some systems, the injector is also heated electrically or with engine coolant (not shown in graphic). There are two NOx sensors with the upstream sensor indicating the level of NOx in the exhaust. This sensor input allows the ECU to determine how much DEF to inject. The downstream NOx sensor measures the efficiency of the SCR catalyst. The upstream exhaust temperature sensor tells the ECU when the exhaust is hot enough for SCR operation, and when to activate the heated catalyst (if equipped). The downstream temp sensor measures SCR catalyst efficiency.

Figure 6 - BlueDEF Diesel Exhaust Fluid is formulated to meet the stringent requirements of the American Petroleum Institute (API). The company also offers reusable DEF dispensing systems for fleet, retail and commercial applications that provide significant savings over single-use containers.

DEF and vehicle owners

Unlike other emission-related systems on diesel vehicles, SCR requires that the vehicle’s owner do something to keep it operational — ensure that the DEF tank doesn’t run dry. Dealerships and independent repair shops know all too well that it’s a mixed bag when it comes down to drivers reading the section in their owners’ manual about keeping the DEF tank from running low or out of fluid. On most vehicles, DEF range in miles corresponds with engine oil change intervals — usually around 7000 miles. The logic of this design is to have the DEF tank filled when changing engine oil. However, this doesn’t always work out and many owners overlook filling the DEF tank as part of an oil and filter service.

SCR systems don’t kid around when it comes to low, or no DEF fluid. A low level will result in messages sent to the driver via warning lights or text on the vehicle’s digital display. There is no standard among OEMs regarding DEF warnings. On some vehicles, the digital display “Service Vehicle” menu may have a “DEF refill in XXX miles” function that informs owners when to add DEF. Also, OEMs provide a variety of progressive warnings regarding decreasing DEF levels. A low DEF warning, and/or instrument panel light may display around 1500 miles. Other messages may include a text warning "No engine restarts possible soon. Fill DEF tank” and “Distance to NO ENGINE RESTART 100 miles.” Once the “No Engine Starts Possible” warning is displayed the vehicle must be driven to a location where DEF can be added to the tank before the engine is turned off.

If the engine is turned off the ECU will prevent a re-start. Owners that carry a 2.5-gallon jug of DEF may think that it is enough to get the engine re-started but, no (in fact, the fun is just beginning), as many SCR systems require the DEF tank to be “full ‘(another 2.5+ gallons on some vehicles) before allowing engine starting. If the vehicle is driven until the DEF tank is dry, the ECU puts the engine into limp-in mode limiting vehicle speed from 5 to 10 mph. Even if the distance to the nearest truck stop to buy DEF is only a few miles it will be a long trip. Limp-in mode and no engine restarts are mandated by the EPA and show how serious they are about uncontrolled NOx spewing from diesel exhaust.

Common SCR system problems

Figure 7 - The Bosch Automotive Service Solutions OTC 3095 tester simplifies DEF testing. By placing a drop of DEF on the tool’s sensor the percent of urea is displayed. The tool can also measure the freezing point in degrees F of windshield washer fluid.

An often overlooked component of SCR systems is DEF fluid. Automotive-grade urea like BlueDEF, AdBlue, and other brands, are API-certified to contain 32.5 percent urea and 67.5 percent deionized water. When faced with a “DEF Contaminated” code or scan tool message it’s a good idea to find out what is in the DEF tank. Owners have been known to put other liquids into DEF tanks including Dex-Cool antifreeze, diesel fuel, distilled water, tap water, expired/old DEF, and even human urine. Some SCR systems have a DEF quality sensor or use ECU software to indirectly determine DEF quality. For example, the ECU activates the DEF injector to spray fluid into the exhaust. The downstream NOx sensor reports that NOx levels are too high, the ECU injects more DEF into the exhaust but NOx levels don’t drop causing a DEF contamination code or “Exhaust Fluid Quality Poor” message to be displayed.

Figure 8 - The Bosch Automotive Service Solutions OTC 5095 DEF tester is similar to the refractometer tools used to test coolant. After placing a drop of DEF on the tool's prism, the user points the tool toward a bright light and looks through the eyepiece. By adjusting the focus ring, the reticle scale will indicate the percent of DEF to water. 

DEF testing should be done first when diagnosing DEF-related trouble codes. There are several DEF testers available including optical and electronic. A DEF refractometer is an optical tool that uses light to determine the percentage of urea contained in DEF fluid. These tools read the percentage of urea in 0.5 percent increments. Any reading other than 32.5 percent is a good indication that the DEF is contaminated, too old, or not present in the tank. It’s a waste of a technician’s time to chase down multiple SCR-related DTCs if DEF is contaminated. If it is, drain the tank, clean it, purge the lines and injector then refill with fresh DEF.

DEF fluid is unique among automotive products in that it has a shelf life. This is not the norm and technicians would not consider using a three, or more year-old container of engine oil, coolant, power steering fluid, brake fluid (unopened), or other automotive liquids as something to be concerned about. Every container of DEF has a date code, aka “Born-On-Date” or “Sell-by-Date.” A typical code of “GA153590089” can be read where: GA is the plant that made the DEF; 15 is the year of manufacturing plus 1 year (this batch was made in 2014 + one year = 15); 359 is the day of the year (365 - 359 = 6 so this batch was made on the sixth day of the year, or January 6, 2014) and 0089 is the batch code.

Repair shops need to consider the environment where they store DEF. When stored at temperatures above 95º F, the DEF shelf life is shortened to six months and above 104º F to two months. Some customers want to be prepared in case they run low on DEF while on the road, so they keep a container in the trunk or inside the cab of their pickup truck. This is not a good idea as these locations can reach temperatures of over 140º F on a hot summer day. By the time they need the DEF, it will be way past its shelf life due to the high storage temperatures. The BlueDEF spec sheet states that “The shelf life of DEF is directly related to the temperature at which it is stored. Storage temperatures between 12º and 86º F are recommended to maintain the optimal shelf life of up to two years. If BlueDEF freezes, its efficacy will not be effected upon thawing.”

Figure 9 - This Bosch Automotive Solutions NOx sensor and controller are integrated as one part. Because NOx sensors can’t differentiate between NOx readings and ammonia in DEF fluid a malfunction in the SCR system could set a code for a faulty NOx sensor. To read true  NOx sensor values, but the system into regen (cleans out any ammonia in CAT) so scan tool values for NOx will be accurate. Symptoms of a bad NOx sensor are increased fuel consumption and/or rough idle quality.

Most SCR systems don’t require an undue amount of service but some common things can go wrong. There are around 100 OBD-II SCR-related trouble codes and using a scan tool to diagnose SCR problems is, to say the least, helpful. Not listed in any particular order are some basic, common SCR issues provided by the SCM Hotline (a company that for over 25 years has offered automotive diagnostic support to professional technicians). The most common issues are:

-DEF tank/injector heater failures

-Blocked or restricted DEF lines

-DEF injector plugged and contaminated

-Expired DEF fluid. The following are some common SCR trouble codes.

DTC P20EF is defined as, “SCR NOx Pre-Catalyst Efficiency Below Threshold.” When the DEF tank is refilled the SCR Catalyst Efficiency monitor is triggered. After the monitor is complete, and the SCR function is normal the monitor continues to calculate the cumulative efficiency of the SCR system. Each subsequent value for cumulative efficiency is included in two filtering routines, one for short term efficiency and the other for long term efficiency. If the difference between the two filtered efficiencies becomes greater than a pre-set threshold, the P20EF fault is set. Common problems that set a P20EF are DEF contamination, DEF injection failure, faulty NOx sensor(s), and SCR catalyst failure.

Figure 10 - Bosch Automotive Solutions DEF injectors are mounted on the exhaust inlet pipe (just ahead of the SCR catalyst). Notice the heat fins that cool off the injector’s electrical components.

Some other common codes for specific applications are: DTCs P249D and/or P249E on the 6.6L Duramax engine for “Poor DEF” digital display message, incorrect DEF fluid, expired fluid and too sensitive recalibration; The 3.0L Duramax engine may show a P208E for DEF heater electrical connectors; The VW, Touareg 3.0L V6 can set P20EE, P204F or P207F codes for wrong DEF fluid and incorrect urea amount and the Ford 3.0L Powerstroke engine may set a P204F, reductant filter, plugged DEF line or DEF injector.

DEF and diesel fuel mix-ups

Figure 11 - Even though the diesel fuel and DEF filler caps are different colors, sizes and are clearly-marked "Diesel" and "Diesel Exhaust Fluid owners can, and do mix up what goes into each tank. The wrong liquid in either the DEF or diesel fuel tank can cost thousands of dollars in repair bills.

One would think that putting fuel or DEF in the wrong tank would be difficult to do. The filler caps are marked "Diesel" and "Diesel Exhaust Fluid" are different colors and sizes. Still, with these safeguards, owners manage to add DEF fluid to the Diesel fuel tank and vice versa. Either scenario results in huge problems and costly repairs.

What happens if diesel is put into the DEF tank? SCR systems have built-in warnings to detect non-DEF substances. The ECU will signal the driver with a warning and/or code of impending SCR interruption if non-DEF is detected. If contaminated, the DEF tank should be drained and thoroughly cleaned with deionized water before refilling. Because diesel fuel is less dense than DEF it will float on top of the DEF but eventually make its way into the exhaust system. If diesel fuel enters the catalyst it may be damaged to the extent that replacement is the only option. SCR catalysts can cost upwards of $1,000 plus labor.

Worse by far than diesel in the DEF tank is the opposite. The mistake of putting DEF into the fuel tank can cause rough idle, excessive exhaust smoke, low power, engine knocking, fuel rail pressure slow-to-build, or no engine start. If these symptoms weren’t bad enough, DEF in the fuel tank can cost hundreds to thousands of dollars to repair. Because DEF is corrosive it can cause steel lines to rust, fuel injectors to stick open, and high-pressure injection pump failure. The DEF fluid crystallizes and can cause irreparable harm to components that use metals like carbon steel, brass, aluminum, copper, magnesium, nickel, and zinc.

If the owner realizes that they added DEF to the fuel tank, and NOT started the engine, or turned the ignition key on, they could have their vehicle towed to a repair shop. In this case, the fuel tank needs to be drained and cleaned. If the key was turned on, the low-pressure side of the diesel fuel system will have to be cleaned as well. If the engine was started, DEF will be on the high-pressure side of the fuel system. The injectors may need to be removed and cleaned along with all fuel lines; fuel pump flushed and cleaned; fuel filters replaced and the fuel/water separator replaced. The worst-case scenario is when the engine has been operated for some time with DEF present in the fuel tank. The corrosive DEF can damage beyond repair high-pressure fuel injection components. Depending on the vehicle’s age and mileage, these types of expensive repairs could exceed its blue-book value.

Service and repair of SCR system components are another revenue stream for independent repair shops. The use of a scan tool to verify system operation and/or read fault codes is a must. Don’t forget to perform a visual inspection of SCR system components looking for loose electrical connectors and blocked or leaking DEF lines. Because many owners refill their DEF tank there is a real possibility that the fluid is expired so testing DEF fluid quality is a good place to start any diagnosis. Educate your customers about where to store DEF (not in the cab or trunk) to preserve its shelf life. Inform them of the consequences of putting DEF in the fuel tank and vice versa. Finally, explain what running low, or out of DEF entails and encourage them to read their owner’s manual. A small amount of up-front advice will pay off in the long run and keep your customers coming back for more.

About the Author

Tracy Martin

Tracy Martin writes for Motor Age, Aftermarket Aviation Defense, Contact! and Turbo Diesel Register magazines. As an ASE certified master technician and consultant, he has worked in the automotive aftermarket for over 25 years and has provided consulting services for Snap-On Tools, Sun Electric, Chevron Oil and Nissan of North America. He has developed automotive/powersports curriculum and taught courses including: engine performance, automotive electronics, fuel injection, ABS braking systems and advanced motorcycle riding techniques. He has also provided consulting services as an expert witness for automotive and powersports-related court actions.

He is the author of five books: How to Diagnose and Repair Automotive Electrical Systems, Motorcycle Electrical Systems–Troubleshooting and Repair (two editions), How to Use Automotive Diagnostic Scanners (two editions) and How to Modify Motorcycle Fuel Injection, all published by Motorbooks International. 

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