A diesel engine is equipped with a modern DOC/DPF/SCR after-treatment system. The SCR system is experiencing frequent 'NOx Conversion Efficiency Low' fault codes, despite the DEF quality being confirmed good and DEF dosing appearing normal. What is a probable cause for low NOx conversion, considering other components are functional?

Question: A diesel engine is equipped with a modern DOC/DPF/SCR after-treatment system. The SCR system is experiencing frequent 'NOx Conversion Efficiency Low' fault codes, despite the DEF quality being confirmed good and DEF dosing appearing normal. What is a probable cause for low NOx conversion, considering other components are functional?

Answer options: ✅ Degradation or contamination of the SCR catalyst itself, reducing its effectiveness.

• An exhaust leak downstream of the SCR catalyst, affecting sensor readings.
• A faulty exhaust gas temperature (EGT) sensor providing excessively high readings.
• A plugged Diesel Oxidation Catalyst (DOC), causing increased exhaust back pressure.

Correct answer: Degradation or contamination of the SCR catalyst itself, reducing its effectiveness.

Explanation: If DEF quality and dosing are confirmed correct, and NOx conversion remains low, the most likely culprit is the SCR catalyst itself. Over time, SCR catalysts can degrade due to thermal aging, sulphur poisoning, or contamination, which reduces their ability to effectively convert NOx into nitrogen and water, directly leading to 'NOx Conversion Efficiency Low' faults. An exhaust leak downstream of the catalyst would not directly reduce conversion efficiency but could affect sensor readings. Excessive EGT readings would likely cause derate or other faults, and a plugged DOC would primarily cause high back pressure and DPF regeneration issues, not directly low NOx conversion, unless severe for a extended period and contributing to catalyst coating damage.