Emission Control Systems Flashcards
6 cards from real ASE4 practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Emission Control Systems flashcards as text
A knock sensor detects detonation. What does the ECM do in response?
Answer: Retards ignition timing to eliminate detonation
The ECM retards ignition timing when the knock sensor detects detonation, delaying combustion to reduce knock intensity.
The EVAP large leak code P0455 is set. A smoke test finds smoke exiting from around the fuel cap. The correct repair is to:
Answer: Replace the fuel cap and retest
A leaking fuel cap is a common and simple cause of EVAP large leak codes; replacing the cap and verifying the seal corrects the fault.
Which scan tool parameter BEST indicates whether the catalytic converter is working efficiently?
Answer: Comparison of upstream and downstream O2 sensor activity
Comparing upstream to downstream O2 sensor activity reveals how effectively the converter is smoothing exhaust gas composition variation.
A vehicle has high NOx readings on an emissions test but passes HC and CO. The MOST likely cause is:
Answer: A faulty or clogged EGR system
High NOx with normal HC/CO points to insufficient EGR flow, which should be lowering combustion temperatures to suppress NOx.
A continuous EVAP monitor detects a large leak during normal driving. The vent solenoid is commanded closed and the purge solenoid is commanded open. The system then monitors:
Answer: Vacuum decay in the EVAP system
The ECM seals the EVAP system and uses intake vacuum to create a vacuum in the tank, then monitors vacuum decay to detect leaks.
Carbon monoxide (CO) is primarily produced when:
Answer: The engine runs rich, leaving insufficient oxygen to complete combustion
CO is the product of incomplete oxidation of carbon; a rich mixture leaves excess carbon that cannot find enough oxygen to form CO2.