What’s the deal with Diesel Exhaust Fluid?

Regardless of the Volkswagen diesel emissions scandal, diesel engines are making inroads into light truck and car sales. What more and more drivers are learning about every day is one of the emissions-control features of post-2009 model year diesels: Diesel Exhaust Fluid, or DEF.

DEF and its sophisticated control/delivery systems are stored on board to inject this urea-based fluid into the exhaust stream to help lower tailpipe output of nitrogen oxides. Drivers are required to refill their auto’s DEF tank every 4,000 to 10,000 km, depending on vehicle use and load. DEF composition should be near 32 per cent automotivegrade urea and the remainder purified water. An instrument panel display will provide monitoring of the distance remaining to refill time. If the tank is depleted, the vehicle may refuse to restart after a shutdown, making the DEF gauge something a driver doesn’t want to ignore.

One area of concern for many diesel light-vehicle owners is the major cost spread for DEF fluid. Original automaker dealerships will charge US$12 per gallon and up and the average driver can go through several gallons per month. But DEF is also used in heavy trucking, which is why almost all commercial truck refuelling centres offer bulk DEF dispensed through fuelpump type stations. These DEF retailers can offer prices as low as US$3 per gallon.

Passenger-vehicle DEF systems continually monitor (and report via the instrument panel) the operation of this emissionscontrol feature. This includes the ability to warn of poor quality DEF, and may also reduce engine power to limit NOx emissions until the problem is resolved.

In many cases, the system’s sensors or software can just as often be responsible for the fault as less-than-stellar-grade DEF. But when you take your vehicle to a repair centre, be prepared to be charged for the diagnosis and costs of draining and refilling the DEF tank, along with any other repairs.

National-brand commercial refuellers have a lot at stake when it comes to ensuring DEF quality, as they can be held liable for the expenses of heavy truck diagnoses and repairs. But regardless of the reputation of a highway DEF retailer, automaker dealership service departments will usually stand by their position that only their authorized DEF fluid should be used.

If your diesel’s dash pops up a DEF warning, you should first ask your service garage to check for any manufacturer bulletins regarding upgraded software or DEF components that may be related to whatever fault code is stored. Remember, if the fault is traced to impure DEF fluid not sourced from the automaker’s approved supply, you may face diagnostic fees when the vehicle is still within warranty limits. Sometimes draining and refilling the DEF tank from a confirmed good source may rectify the fault, but it’s not always an easy DIY task.

If your ride has had a history of repeated DEF quality faults, you may want to invest in a DEF tester tool (available from many auto-parts stores).

The tester is a refractometer than can quickly verify DEF condition with a few drops of the liquid. Or better yet, if you can connect DEF dash warnings to particular DEF suppliers, avoid those in the future.