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
What is the correct interpretation of a downstream (post-catalyst) O2 sensor that mirrors the upstream sensor's oscillation?
Answer: The catalytic converter is not efficiently storing and releasing oxygen, indicating it has failed
A healthy catalytic converter buffers oxygen, causing the downstream sensor to hold a steady mid-voltage; if it mirrors upstream oscillation, the converter has lost its oxygen storage capacity.
A vehicle has a rotten egg smell from the exhaust. What is the cause?
Answer: Hydrogen sulfide produced when the catalytic converter processes sulfur-containing fuel
Catalytic converters can convert sulfur dioxide from gasoline into hydrogen sulfide (H2S) gas, which has the characteristic rotten egg odor, particularly when running rich.
A vehicle equipped with SCR (Selective Catalytic Reduction) has a warning about DEF (diesel exhaust fluid) quality. What effect will poor DEF quality have?
Answer: Reduced NOx conversion efficiency and possible system shutdown
SCR systems require a precise 32.5% urea solution; incorrect concentration reduces the chemical reaction efficiency that converts NOx into harmless nitrogen and water.
An exhaust back pressure test shows pressure above 1.5 psi at idle. What does this indicate?
Answer: A restriction in the exhaust system such as a clogged catalytic converter or muffler
Back pressure above approximately 1–1.5 psi at idle indicates a restriction that will worsen under load, reducing engine power and potentially causing overheating of the converter.
A vehicle fails the OBD-II portion of an emissions test because the readiness monitors are not set. What is the most likely reason?
Answer: The battery was recently disconnected, clearing the PCM memory and resetting the monitors
When battery power is lost or the PCM is reset, all OBD-II readiness monitors are cleared; the vehicle must be driven through specific drive cycles for each monitor to run and complete its self-test.
What is the purpose of the oxygen storage capacity (OSC) in a catalytic converter?
Answer: To buffer mixture fluctuations by absorbing excess oxygen during lean conditions and releasing it during rich conditions
OSC allows the converter to continue oxidizing HC and CO even when the air-fuel mixture briefly goes rich by releasing stored oxygen, smoothing out the effects of mixture oscillations.