Diesel vs. gas: Weigh cost of DEF repairs

By now, most diesel engine enthusiasts know about diesel exhaust fluids (DEF) emissions control systems. For many light use applications, drivers only have to refill their DEF tanks on a monthly basis. Some leave it to their service providers to top it off during regular oil changes.

The rate of consumption is determined on engine load and can vary wildly between similar vehicles. This minor chore for most is offset by predictable and consistent fuel economy and the benefit of increased torque from the drivetrain, making towing and carrying heavier loads a breeze.

Getting the same amount of pulling power from a gasoline power plant usually entails much larger displacements or turbocharging, neither of which lead to better fuel mileage. But with new diesels, there are trade-offs; the main compromises are the cost of maintenance and repairs, as compared to traditional gasoline vehicles.

We covered the risks of using poor quality DEF fluid in earlier columns and provided warnings to drivers regarding the costs of diagnosing check engine lights that are caused by this fluid when it contains anything more than trace amounts of hydrocarbons. Dealership service departments routinely charge their clients for time spent on tracing a warning light down to contaminated DEF, even when these vehicles are still within the time and distance limits of their warranty. Automakers expect these facilities to have chemical test strips on hand to determine the quality of DEF in their products. (Read the fine print on your warranty documents and you’ll find these shops are justified in this practice.)

But it’s not only poor quality DEF that causes problems. Poor designs, build quality and temperamental weather also take their toll.

DEF systems have built-in heaters to keep the fluid in liquid form because it can freeze, damaging pumps and injectors, but Mother Nature has a way of beating the best laid plans of mice and diesel engineers. When heaters are placed in the area of a DEF storage tank, some of it can still freeze or thicken, leading to injection problems. Some injectors can cost more than $1,000 just for the part, not to mention diagnosis and replacement costs. Tank modules can run $2,000 or more. And then there’s the myriad sensors, relays, controls, lines and such to consider. For Fiat Chrysler Automobiles, Volkswagen and Mercedes-Benz (for example), these systems are only covered by the base warranty of three or four years (depending on vehicle manufacturer).

As all DEF-using vehicles have control systems to shut the engine down (or prevent it from restarting) when major failures occur, ignoring warnings won’t lead to happy results. And while some heavy-truck service centres have been reported to have developed software bypasses, few of these illegal cures exist for passenger units.

The price points of DEF system repairs has become enough of an issue with FCA that it has extended coverage for most DEF system parts for eight years, but only on 2018 and later vehicles. As FCA recently announced its decision to cease sales of lightduty diesels in North America (no firm date yet announced) this won’t be much of an additional risk to its bottom line. Mercedes also reported in 2017 that its four-cylinder diesels were “off the table” because of the difficulty in achieving certification in North America.

So when determining the risk versus benefit for diesel over gasoline, take into account the costs of DEF-system repairs after the warranty expires. Take a stroll to the parts and service counters when shopping in a dealership to ask their opinions on repair expenses and what to expect down the road. If you suspect less-than-forthright answers, check things out with an independent shop you know and trust.