DFFT Exhaust and Emissions 2 — Questions and Answers
Question 1: A P0420 code (catalyst efficiency below threshold) is stored. Before replacing the catalytic converter, what should be done?
- Diagnose and repair any misfires, rich conditions, or O2 sensor faults that could damage or misdiagnose the converter (Correct answer)
- Test exhaust back pressure to confirm the converter is clogged
- Check the exhaust manifold for cracks causing air leaks
- Verify the fuel grade being used is correct for the vehicle
Correct answer: Diagnose and repair any misfires, rich conditions, or O2 sensor faults that could damage or misdiagnose the converter
A P0420 can be caused by a genuinely failed converter, but it can also be caused by contaminated or improperly functioning oxygen sensors or upstream engine issues that damage the converter.
Question 2: A diesel vehicle has excessive black smoke at all operating conditions. What is the most likely cause?
- Over-fueling from a faulty injector, high fuel pressure, or a boost pressure fault (Correct answer)
- A clogged diesel particulate filter reducing exhaust flow
- A cracked exhaust manifold allowing air to enter the exhaust
- A faulty diesel oxidation catalyst
Correct answer: Over-fueling from a faulty injector, high fuel pressure, or a boost pressure fault
Persistent black smoke on a diesel means excessive unburned fuel at all loads, which is caused by over-fueling due to injector faults, incorrect timing, or insufficient air from a boost failure.
Question 3: A diesel particulate filter (DPF) becomes prematurely clogged. What is the most common operational cause?
- Frequent short trips that prevent the DPF from reaching regeneration temperature (Correct answer)
- Using low-sulfur diesel fuel in a high-sulfur DPF system
- Running the engine at maximum load continuously
- Using the wrong grade of motor oil causing excessive ash
Correct answer: Frequent short trips that prevent the DPF from reaching regeneration temperature
DPF regeneration requires sustained exhaust temperatures above approximately 600°C, which is only achieved on longer drives; constant short trips allow soot to accumulate without burning off.
Question 4: What does a wideband (UEGO) oxygen sensor measure that a traditional narrowband sensor cannot?
- The actual air-fuel ratio across a wide range, not just rich or lean (Correct answer)
- Exhaust temperature downstream of the catalytic converter
- The oxygen content specifically in the exhaust catalyst bed
- The switching speed between rich and lean mixture states
Correct answer: The actual air-fuel ratio across a wide range, not just rich or lean
A wideband sensor produces a linear voltage or current output proportional to the actual lambda value, enabling the ECU to precisely target any air-fuel ratio rather than just toggling around stoichiometric.
Question 5: Blue-gray smoke from the exhaust at all operating temperatures indicates what condition?
- Engine oil being burned in the combustion chambers (Correct answer)
- Unburned diesel fuel in the exhaust stream
- Coolant entering the combustion chamber and vaporizing
- Rich fuel mixture during cold start warm-up
Correct answer: Engine oil being burned in the combustion chambers
Blue smoke is burning oil; it can be caused by worn valve stem seals, worn piston rings, or a failed PCV system allowing oil vapor to enter the intake.
Question 6: A vehicle's secondary air injection system injects air into the exhaust manifold during cold starts. What is its purpose?
- To combust HC and CO in the exhaust before the catalytic converter reaches operating temperature (Correct answer)
- To cool the exhaust manifold and prevent warping during cold starts
- To purge residual exhaust gases from the manifold before ignition
- To provide extra oxygen to the catalytic converter during regeneration
Correct answer: To combust HC and CO in the exhaust before the catalytic converter reaches operating temperature
During cold start the catalytic converter is below its light-off temperature; injecting air causes additional oxidation of HC and CO in the hot exhaust manifold to reduce emissions during warm-up.
DFFT Exhaust and Emissions 2
A P0420 code (catalyst efficiency below threshold) is stored.
Before replacing the catalytic converter, what should be done?