Emissions Control Systems Diagnosis and Repair
What the candidate must understand
Modern diesel emissions control includes EGR, DOC, DPF, SCR, and associated sensors and dosing systems. Know the purpose of each: EGR reduces NOx by lowering combustion temperature; DOC oxidizes CO and HC and helps regenerate the DPF; DPF traps soot and regenerates by heat; SCR uses DEF and a catalyst to reduce NOx. Understand that EGR cooler leaks cause coolant loss and white smoke, and that a stuck EGR valve causes rough running, low power, or excessive smoke.
Know DPF regeneration types (passive, active, forced) and the conditions required: temperature, duration, and vehicle speed. Understand that short-trip operation can prevent regeneration and cause soot loading. Know DEF quality, level, and dosing faults, and that derates protect the system. Understand sensor roles: NOx sensors, DPF pressure sensor, and temperature sensors.
Traps
Questions often confuse EGR and SCR roles, or blame the DPF for a problem caused by a failed EGR cooler. Another trap is assuming a DPF code means replacement; often a forced regeneration or sensor repair solves it. Watch for answers that ignore DEF quality and level.
Worked example / rule of thumb
Rule of thumb: follow the soot and the NOx. Black smoke and high DPF pressure suggest over-fueling or EGR faults; a NOx efficiency code with good DEF suggests an SCR or NOx sensor issue. Confirm DEF quality and level before condemning the SCR catalyst.
Key points
- EGR lowers combustion temperature to reduce NOx.
- DOC/DPF handle soot and oxidation; SCR uses DEF for NOx.
- EGR cooler leaks cause coolant loss and white smoke.
- Regeneration needs temperature, time, and speed; short trips cause loading.
- Check DEF quality/level and sensors before replacing SCR or DPF.