A 2020 off-highway mining truck equipped with a Selective Catalytic Reduction (SCR) system is frequently displaying a 'DEF Quality Low' fault code. The operator reports refilling the Diesel Exhaust Fluid (DEF) tank from a bulk supply. What is the most probable issue?

Question: A 2020 off-highway mining truck equipped with a Selective Catalytic Reduction (SCR) system is frequently displaying a 'DEF Quality Low' fault code. The operator reports refilling the Diesel Exhaust Fluid (DEF) tank from a bulk supply. What is the most probable issue?

Answer options: ✅ Contamination of the bulk DEF supply with diesel fuel or water.

• A faulty DEF level sensor in the truck's DEF tank.
• A clogged DEF doser valve, restricting fluid delivery.
• An excessively high engine exhaust gas temperature sensor reading causing incorrect DEF dosing.

Correct answer: Contamination of the bulk DEF supply with diesel fuel or water.

Explanation: Contamination of the Diesel Exhaust Fluid (DEF) with impurities like diesel fuel or water will significantly alter its urea concentration and chemical properties. The SCR system's DEF quality sensor will detect this deviation from specification (typically 32.5% urea), triggering a 'DEF Quality Low' fault and potentially leading to engine derate as per Tier 4 emission standards. Faulty DEF sensors would typically show level issues, not quality.