If your operation’s shop routinely services diesel-equipped school buses and other diesel-powered vehicles, you may be aware of the use of DEF (diesel exhaust fluid). DEF consists of a mixture of high-purity urea and deionized water. Controlled by a “dosing” module, DEF is injected into the diesel engine’s exhaust stream. DEF consists of a mixture of 32.5% high-purity synthetic urea and 67.5% deionized water.

DEF dosing is controlled by the engine’s ECU. When heated, DEF splits into ammonia and carbon dioxide, which is then atomized and vaporized. Once DEF enters the exhaust, the water in the DEF vaporizes, leaving ammonia molecules to travel to the catalytic converter, where it neutralizes NOx molecules. This reaction converts NOx to harmless nitrogen and water, substantially cleaning up diesel emissions.

The EPA has mandated that all on-road diesel-equipped vehicles manufactured after January 2010 must reduce NOx emissions. The most common method to achieve this reduction is by injecting DEF into the exhaust path. All diesel-equipped vehicles that feature an SCR (selective catalytic reduction) system utilize DEF injection.

The use of SCR systems reportedly provides an additional benefit of a 3% to 5% increase in diesel fuel economy. This is a result of enhanced combustion as exhaust temperature rises, especially in the heavy-duty applications. Cummins reportedly has gained substantial mileage improvements with its SCR system.

DEF usage rate
When the DEF onboard tank is approaching empty, the vehicle’s onboard warning system will alert the driver with enough lead time to replenish the DEF tank.

Instead of (or in addition to) refilling DEF tanks when an early warning is provided, it’s wise to top off a DEF tank at the same time that the engine oil is changed (for example, every 3,000 to 4,000 miles). In other words, you need to keep on top of the DEF level status. Running the system dry will result in severe driver inconvenience, in terms of entering a limp mode (either loss of power or severely limited vehicle speed).

DEF tank fill location
All DEF fill caps, regardless of make or model, should be light blue in color and should be labeled “DIESEL EXHAUST FLUID.” When filling the DEF tank, do not overfill. In freezing temperatures, the DEF mixture will expand. DEF tanks are designed to accommodate this expansion, but only if you fill to the advised level. If a DEF tank does become damaged and leaks, there’s no real hazard since it’s not toxic or flammable. It’ll just make a stinky ammonia-smelling mess.

Once DEF temperature reaches approximately 12 F, it will freeze. However, this won’t leave the vehicle stranded. The engine is able to start, and the vehicle may be driven normally. As engine coolant warms, the DEF will thaw and will once again flow properly. While there have been claims that urea will become toxic at 118 F, this isn’t so. Actually, urea isn’t toxic at any temperature. If it becomes too hot, the shelf life may simply decrease (as ammonia begins to form).

Do not add any additives, such as antigel, to any DEF tank. DEF will expand by about 7% when frozen and will revert to a liquid state when heated during engine operation. Don’t treat DEF as you would diesel fuel. You don’t need, nor should you ever add, any modifiers/additives. Keep the DEF pure!


Misco offers digital refractometers. The model 201 refractometer offers a single-scale readout. Photo courtesy of Misco

Misco offers digital refractometers. The model 201 refractometer offers a single-scale readout.
Photo courtesy of Misco

Home-brew alternatives?

Just because the DEF solution comprises a mixture of urea and water, this does not mean that you can whip up your own batch. The urea used in DEF is a very high-purity synthetic, and it is not urine. Also, the water used in DEF must be extremely pure and deionized. The bottom line: No, you can’t make your own DEF by mixing urine and tap water. DEF is a sophisticated blend designed specifically for SCR systems.

Allowing impurities such as calcium, copper, magnesium, etc. (which would/could be present in urine and tap water), would introduce impurities that can kill active sites in the catalytic converter (the ammonia itself won’t harm the sites). Continued use will eventually make the converter inoperable, requiring converter replacement.

Also, do not add anything to the DEF tank except actual DEF. If someone, by accident or ignorance, dumps diesel fuel treatment/booster into the DEF tank, this will kill the entire SCR system, resulting in a repair bill of thousands of dollars. Pay attention. You wouldn’t add orange juice to your fuel tank or Pepsi to your cooling system, so don’t add anything but the correct fluid to the DEF tank.

Along the lines of DEF purity, consider the quality of the DEF. The major brands (Peak’s BlueDef, ACDelco, Motorcraft, etc.) offer the highest quality both in terms of purity and proper urea-to-water percentages. However, there are a few bargain-priced brands out there that feature a higher water content, which isn’t as efficient and won’t last as long as anticipated. In other words, stick with the major brands.

Keep it clean
Whenever handling DEF, the fluid must be kept clean. If a funnel is used, it must be absolutely clean and free of any contaminants. It’s best to dedicate a funnel (and label it) for DEF only. Spare DEF should be stored in its original container, with the lid/cap secured, to avoid airborne contaminants in the shop. Do not transfer DEF to another container that previously held anything else (oil, windshield wiper fluid, coolant, etc.). Avoid cross-contamination! It doesn’t take any extra time to handle DEF properly.

FYI: Urea is corrosive to metals such as copper and brass. DEF systems are typically plumbed with high density polyethylene.

Verifying DEF quality
All API (American Petroleum Institute) certified DEF will feature an API symbol on the container. Use only API certified DEF.

The concern with DEF quality (beyond API certification) is the potential for water dilution. Just as water can enter a fuel system, it is possible for water (moisture) to enter the SCR system and slightly dilute the DEF. One indication that the DEF has been diluted is when the vehicle uses more DEF than normal (beyond the mileage use specified by the vehicle maker).

An easy field test to check for the purity of DEF is with the use of a hand-held refractometer. A refractometer that is designed for testing DEF will easily allow you to verify the water content.

A refractometer is a precision optical instrument designed to measure the concentration or mixture of water-soluble fluids. It measures refractive index, which is the speed at which light passes through a liquid. The more dense the liquid, the slower light will travel through it, resulting in a higher reading.

Applying a few drops of the vehicle’s DEF to the refractometer is all that’s needed. DEF refractometers (those units capable of measuring urea concentration) are available with single scale or dual scale readouts. The single scale provides a percentage of urea by weight. A dual scale unit also provides a refractive index scale readout, which is primarily used by manufacturers and distributors of DEF (a quality control aid). For shop technicians who simply want to check for water dilution, a single scale unit will suffice.

Citing two examples of DEF-capable refractometers, MISCO U.S. offers a digital single scale unit (its DEF-201) that retails for around $350. The reading will provide a precise level of water content to +/– 0.1%. The dual scale unit (DEF-202) retails for around $435. Either model is powered by a pair of AA batteries.

OTC (Bosch Automotive Service Solutions) has released the OTC-5025 DEF refractometer, which retails for around $114. The kit includes a refractometer, dropper, lens wipe cloth, instructions, screwdriver and storage case. The refractometer features a reticle (scale) that is enlarged through an eyepiece to measure light. The values on the scale have been established to specifically evaluate DEF condition. OTC’s unit features 0.5% line graduations, with a scale range of 15% to 40%.

Regardless of the brand you choose, a DEF-capable refractometer is a useful and must-have checking tool for any shop that services diesel-powered vehicles. This eliminates guesswork with regard to the quality (in terms of water dilution) of customer DEF tank contents.


OTC (Bosch Automotive Service Solutions) offers an affordable, high-quality DEF refractometer with its model 5025. Photo courtesy of Bosch Automotive Service Solutions

OTC (Bosch Automotive Service Solutions) offers an affordable, high-quality DEF refractometer with its model 5025.
Photo courtesy of Bosch Automotive Service Solutions

Storing DEF
Storage temperatures need to be held at between 12 F and 86 F. According to sources, this maintains optimal shelf life for up to two years. If DEF is allowed to freeze, once thawed, its efficiency returns to normal. Prolonged storage above 86 F will cause hydrolysis to occur, reducing shelf life.

DEF is available in a variety of volumes, including 1-gallon containers, 2.5-gallon containers, 55-gallon drums and 275-gallon totes.

DEF is also available at the pump, primarily at truck stops and diesel repair facilities.

Our understanding at this time is that DEF is sold in North America as DEF, Peak’s BlueDef, Valvoline’s Airshield, C-Blue, TerraCair and Air1.

Mike Mavrigian is editor of Auto Service Professional, a sister magazine of SCHOOL BUS FLEET, published by Bobit Business Media. This article originally appeared in the May/June 2013 issue of Auto Service Professional.