Exhaust and Emissions Flashcards
6 cards from real DFFT practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Exhaust and Emissions flashcards as text
A P0420 code (catalyst efficiency below threshold) is stored. Before replacing the catalytic converter, what should be done?
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.
A diesel vehicle has excessive black smoke at all operating conditions. What is the most likely cause?
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.
A diesel particulate filter (DPF) becomes prematurely clogged. What is the most common operational cause?
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.
What does a wideband (UEGO) oxygen sensor measure that a traditional narrowband sensor cannot?
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.
Blue-gray smoke from the exhaust at all operating temperatures indicates what condition?
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.
A vehicle's secondary air injection system injects air into the exhaust manifold during cold starts. What is its purpose?
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.