The Tier 4 Final / Stage V emissions systems on modern excavators and loaders are engineered to run on Ultra-Low Sulfur Diesel (ULSD), which has a maximum of 15 parts per million (ppm) of sulfur. But in remote regions or developing nations, contractors are still buying cheap, high-sulfur "off-road" or agricultural diesel that can contain 500 to 5,000 ppm of sulfur. They are saving pennies per gallon, and destroying their aftertreatment systems in the process.
The most vulnerable component is the NOx sensor. These sensors use a precious-metal ceramic element that measures nitrous oxides in the exhaust stream to tell the ECU exactly how much DEF to inject. When high-sulfur fuel combusts, it creates sulfur dioxide, which converts to sulfuric acid in the exhaust. This acid permanently poisons the ceramic element of the NOx sensor.
When the NOx sensor dies, it usually reads artificially high. The ECU thinks the engine is producing massive amounts of NOx, so it commands the DEF doser to inject maximum urea. This massive over-injection of DEF causes the exhaust gas temperature to plummet, which leads to severe DEF crystallization and plugged SCR catalysts. A single tank of high-sulfur fuel can wipe out a $1,500 NOx sensor in 50 hours. The contractors then bypass the sensors with cheap emulator boxes, which works temporarily until the DPF soot load becomes uncontrollable and the engine grenading from exhaust backpressure.
