Emissions
Which of the following components is responsible for reducing NOx emissions in a modern diesel engine?
Selective Catalytic Reduction (SCR) System
Technician A says that the EGR system helps reduce NOx emissions by lowering combustion temperatures. Technician B says that the EGR system increases engine power. Who is correct?
Technician A only
A diesel truck is showing a DPF regeneration warning. What is the most likely cause?
Excessive soot accumulation in the DPF
A diesel engine exhibits poor fuel economy and elevated NOx emissions. The EGR valve appears to be functioning. What is the MOST likely cause?
EGR cooler is partially restricted
Explanation: A restricted EGR cooler limits exhaust gas flow, reducing NOx control and increasing combustion temperatures, which hurts fuel economy.
A diesel engine is setting a code for “EGR cooler efficiency below threshold.” What is the MOST likely cause?
Coolant flow through the cooler is restricted
Explanation: The EGR cooler relies on coolant to reduce exhaust gas temperature. Restricted flow reduces cooling efficiency, triggering a fault.
What is the primary function of the Diesel Oxidation Catalyst (DOC)?
Oxidize hydrocarbons and carbon monoxide
A diesel engine equipped with SCR is using more DEF than usual. Which of the following is the MOST likely cause?
Faulty DEF injector
A diesel engine is experiencing frequent derates and a code for “DEF quality below threshold.” Which of the following tests should be performed FIRST?
DEF refractometer test
Explanation: DEF quality issues are often due to contamination or dilution. A refractometer test quickly verifies urea concentration.
A diesel engine with an SCR system is setting a fault code for NOx efficiency below threshold. Which of the following is the MOST likely root cause?
DEF fluid contaminated with water
Explanation: Contaminated DEF can reduce the effectiveness of NOx conversion in the SCR catalyst, triggering efficiency codes.
How Many Sensors Does the DEF system Have? Name them all
Capacitive, Thermistor (NTC), Piezoelectric, Ultrasonic.
A diesel engine is experiencing poor throttle response and black smoke under load. The DPF is recently replaced. What should be checked NEXT?
EGR valve for sticking
Explanation: A stuck EGR valve can cause excessive soot and poor combustion, leading to black smoke and sluggish performance.
A technician is troubleshooting a diesel engine with frequent passive regenerations and high fuel consumption. What is the MOST likely cause?
DOC is not reaching light-off temperature
If the DOC doesn’t reach the required temperature, passive regeneration is ineffective, leading to frequent active regens and increased fuel use.
A technician is performing a manual regeneration on a diesel truck. The regeneration fails to complete, and the ECM logs a low DOC inlet temperature code. What is the MOST likely cause?
Faulty fuel dosing valve
Explanation:The fuel dosing valve helps raise exhaust temperatures for regeneration. If it fails, the DOC won't reach the required temperature.
A technician is diagnosing a diesel engine with frequent active regenerations. The DPF differential pressure sensor reads within spec, but the soot load is still high. What should be checked NEXT?
DOC inlet temperature sensor
Explanation: If the DOC inlet temp sensor is faulty, the ECM may not initiate or complete regeneration properly, leading to soot buildup.
Name all the systems in the Diesel Emissions Systems by the full name.
EGR Valve, DOC, DPF, SCR
A diesel engine with a DPF and SCR system is showing a code for “excessive DEF consumption.” What is the MOST likely cause?
NOx sensor upstream of SCR is biased low
Explanation: If the upstream NOx sensor under-reports NOx levels, the ECM may overcompensate by injecting more DEF than necessary.
A diesel engine is experiencing white smoke during cold starts, which clears up after a few minutes. No fault codes are present. What is the MOST likely cause?
Glow plug system malfunction
Explanation: White smoke on cold starts often indicates incomplete combustion due to poor cylinder temperature, commonly caused by faulty glow plugs.
A diesel engine is setting a code for “EGR flow insufficient.” The EGR valve is new and functioning. MAP and MAF sensors are reading correctly. What is the MOST likely cause?
Clogged EGR cooler
Explanation: A restricted EGR cooler can limit exhaust gas flow, even if the valve and sensors are working properly.
During a forced regeneration, the technician notices that the DPF outlet temperature is significantly lower than the inlet temperature. What does this MOST likely indicate?
DPF is cracked or has internal damage
Explanation:A damaged DPF may allow exhaust to bypass the filter media, preventing proper heat buildup and soot oxidation.
Name all the different types of fuel injectors.
Mechanical, HUEI, EUI, Common Rail.
During a diagnostic scan, the technician finds that the DPF pressure differential is low, but the soot load is high. What does this MOST likely indicate?
DPF is cracked or bypassed
Explanation: A low pressure differential with high soot load suggests exhaust is bypassing the filter media, possibly due to internal damage.
A diesel engine with an SCR system is showing high NOx emissions despite proper DEF dosing. The upstream NOx sensor reads normal, but the downstream NOx sensor shows elevated levels. What is the MOST likely cause?
SCR catalyst degradation
Explanation:If DEF dosing is correct but downstream NOx remains high, the SCR catalyst may have lost its ability to convert NOx effectively.
A diesel engine with a DPF and SCR system is showing a code for “DPF regeneration incomplete.” The technician verifies that the DPF pressure sensor and temperature sensors are working. What should be checked NEXT?
Turbocharger vane position
Explanation: Proper turbo vane control is essential for achieving the exhaust temperatures needed for regeneration.
A diesel engine with a variable geometry turbo (VGT) and EGR system is setting a code for low EGR flow. The EGR valve is commanded open and responds correctly. What should be tested NEXT?
Turbo vane actuator
Explanation:If the VGT vanes are not positioned correctly, exhaust pressure may be too low to drive EGR flow, even if the valve is open.
A diesel engine is in derate mode due to a “DPF pressure sensor circuit high” code. The sensor has been replaced, but the code persists. What should be checked NEXT?
Sensor wiring and connectors
Explanation: A persistent circuit code after sensor replacement usually points to wiring faults, such as open circuits, shorts, or poor grounds.