DEF Sensors and the DEF Sensor Shortage

Christian Spaccarotella
8 Minutes

DEF Sensors and the DEF Sensor Shortage

First, Diesel Engine Fluid (DEF) sensors are 3 sensors put into 1 apparatus. The DEF level, quality, and temperature are all read via the DEF sensor. For most tractors they are also the source for derate messages these days. "Engine derating is the reduction of an engine’s output due to less-than-ideal operating conditions" according to AMOT Controls Corp. The DEF sensor is the point at which the Environmental Protections Agency (EPA) chose to derate the truck in regards to the after-treatment not working. As a response to the Los Angeles smog problem, in 1968 the EPA introduced the first automobile emissions standards to limit the amount of emissions industries, engines and commercial operations were producing. The solution was to create an after-treatment system that would alter the emissions that were being released by diesel engines. Emissions standard have changed over the decades, but currently everything produced after January 2010 must meet the EPA's emissions standards.

DEF Sensors and Tanks

The main components to an after-treatment system are the DEF Tank assembly, the DEF injector, and an SCR Catalyst. DEF is comprised of 32.5% urea and 67.5% de-ionized water which is injected into the exhaust stream of a diesel engine to reduce the harmful NOx emissions.

DEF Sensors and Tanks

KUS Manufacturer mentions that DEF is nontoxic and nonflammable, but it is corrosive with some metals. DEF tanks are designed specifically with that in mind, usually made from high-density polyethylene. If DEF comes into contact with metals that contain copper or zinc-coated steels, it will lead to contamination and damage the SCR system.

DEF should be stored at a temperature between 75-77 degrees Fahrenheit for the best performance, because if exposed to direct sunlight or it reaches a higher temperature for an extended period of time, it will degrade faster. Colder temperatures will not affect degradation. DEF's shelf life is typically only 2 years old if kept at proper conditions.

To alert drivers when the tank is almost empty, manufacturers are required to include a warning system that shows exactly what is left in the tank. The DEF level sensor is used to indicate how much DEF is left in the tank, and when low it sends a signal to the ECU to show the warning lights. The DEF level sensor is a part of the DEF sending unit, which does more than measure liquids. Inside of after treatment system are two other sensors. The DEF Quality sensor and the DEF Temperature sensor. The quality of the DEF fluid should be 32.5%. If it's under that, more DEF concentration is needed. OTC Tools states, "If concentration level is incorrect, either DEF was of low quality or the tank has become contaminated".

DEF Sending Units

The sending units are mounted to the DEF tank. It shows DEF levels but also DEF temperature, and acts as a temperature regulator. Along with these is a urea quality sensor to measure DEF concentration. KUS explains that "the quality sensor alerts the user if the solution deviates from the acceptable concentration range or if there are any contaminants present in the DEF tank".

SCR System (Selective Catalytic Reduction)

The makeup of an SCR System is the DEF injector, a DEF Tank assembly, and an SCR Catalyst. Diesel Technology Forum explain what the SCR system does is "injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine". That liquid is DEF, which sets off a chemical reaction to convert NOx into Nitrogen, water and CO2. The SCR system is "designed to permit nitrogen oxide reduction reactions to take place in an oxidizing atmosphere". The DEF is the reductant and the SCR system can achieve NOx reductions up to 90%.

Why is there a DEF Sensor Shortage?

The pandemic caused factories to shut down worldwide, including factories that produce urea, rubber, and DEF sensor chips. The demand for urea, rubber, and chips have drastically increased due to trucks' sensors and chips failing. Trucking and the Transportation industry isn't the only industry wanting access to these goods. Agriculture both uses and creates urea, but the pandemic has also widely affected their production. The increased demand and price for urea worldwide has skyrocketed the price of DEF, nearly a spike of 45% according to Southern Counties Lubricants.

The DEF quality sensor shortage is a global one and it has left many drivers off the road. With DEF quality sensors failing and chips failing, the engines are no longer operational as the sensors with derate the engine. Carriers have reported the delays going into June, but a possible solution trucking companies and organizations have talked about a software fix. Some carriers have provided this software fix solution, which "circumnavigate(s) the DEF sensor failure readings to allow the engine to operate normally". The fix is still awaiting approval by the Environmental Protection Agency, and would only be a temporary solution.

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