DEF Sensor Shortage: How To Avoid Trucking Problems

By March 28, 2022Truck Blog

The trucking sector, such as truck dealers in Oklahoma City, has been hard hit by supply chain problems, which can be seen in the new vehicles that are slowly coming off the assembly line. The feared microprocessor shortage isn’t going away, but although there’s been a lot of publicity about not being able to purchase new trucks, there’s been less awareness about the issue that’s putting current vehicles out of commission: the DEF sensor scarcity.

All sorts of products have been impacted by supply chain interruptions, including the chemical urea, which is one of the key components in diesel exhaust fluid (DEF). The difficulty for the trucking business is that without DEF, vehicles would shut down. Therefore, drivers should top off their DEF tanks as soon as possible.

“Due to the outages, we are encouraging drivers to replenish their DEF tank every time they fill their diesel tank,” Chris Hough, vice president of maintenance services for Penske Truck Leasing, stated.

DEF is available in a variety of forms, including bulk DEF from fuel islands and 2.5-gallon containers. The selective catalytic reduction (SCR) feature requires DEF, which is kept in a separate gasoline tank on the vehicle.

DEF is injected into a diesel vehicle’s exhaust stream, where it combines with a catalyst to convert NOx emissions to nitrogen and water. “Without DEF, the SCR system will be unable to convert the NOx emissions, and the vehicle will begin to de-rate,” Hough explained.

According to the EPA, all of the software solutions offered to the agency by engine manufacturers have been authorized, and many of them have already been implemented. Each manufacturer has a number of engine families, each of which needs a different software solution.

Although emergency/temporary software upgrades have been approved and deployed to solve sensor components’ scarcity concerns caused by chip shortages, the SCR emission system continues to work as intended. “There is a common misperception that the emergency/temporary software updates deactivate the SCR system, which is incorrect; the SCR system continues to function as intended, with the exception of the dash DEF-level gauge,” Hough stated.


The semiconductor shortage has had a substantial impact on our supplier’s ability to deliver the number of sensors required to support unit-down circumstances, as it has for the whole industry; suppliers will continue to be at risk until the chip shortage issue is rectified.


It’s difficult to pinpoint a single cause why sensors fail. It can vary by manufacturer, application, and operating environment, to mention a few criteria, as with most things in the fleet sector. When a sensor fails, the response might range from a warning light to a complete engine shutdown.

DEF Quality Sensors (DEFQS) failure can result in an engine torque derate in a variety of ways, including improperly or inappropriately detecting poor fluid quality in the diesel exhaust fluid tank, failing to communicate with the ECM, or being in a failed or offline condition. The longer any breakdown continues in disrepair, the worsening of this derate will get, finally rendering the vehicle undriveable. 


Preventive maintenance is your bestest mate in maintaining your vehicle’s functioning, whether you’ve had DEFQS problems or not.

It is excellent practice for technicians to verify the purity of DEF using a refractometer while vehicles are in the shop for a PM. Doing this and recording the measurements assists fleets and/ or Used Delivery Vans in correlating this data with RO history and determining whether foreign contamination has entered the tank at any stage. Fleets should always use high-quality DEF fluid from a reputable provider to extend the life of their DEF/SCR systems.


Sensor shortages are causing trucking companies across the country to experience problems with their vehicles. DEF, or Diesel Exhaust Fluid, is necessary to control emissions from trucks, and a shortage of the fluid is causing trucks to break down and be unable to meet emissions standards. There is no easy solution to this problem, but by following our blog, you can stay up to date on the latest news and learn how to avoid trucking problems.