ASE4 Emission Control Systems 1 — Questions and Answers
Question 1: The EGR (Exhaust Gas Recirculation) system primarily reduces which exhaust emission?
- Hydrocarbons (HC)
- Carbon monoxide (CO)
- Oxides of nitrogen (NOx) (Correct answer)
- Carbon dioxide (CO2)
Correct answer: Oxides of nitrogen (NOx)
EGR introduces inert exhaust gases into the intake to lower peak combustion temperatures, which reduces NOx formation.
Question 2: A stuck-open EGR valve at idle will cause:
- Increased NOx emissions
- Rough idle and possible stalling (Correct answer)
- Rich air/fuel mixture at idle
- Increased engine power
Correct answer: Rough idle and possible stalling
An EGR valve that stays open at idle dilutes the combustible mixture with exhaust gas, causing rough idle and potential stall.
Question 3: The EVAP (Evaporative Emission Control) system is designed to:
- Reduce NOx emissions from combustion
- Prevent fuel vapor from escaping to the atmosphere (Correct answer)
- Cool the catalytic converter
- Recirculate exhaust gases
Correct answer: Prevent fuel vapor from escaping to the atmosphere
The EVAP system captures fuel vapors from the fuel tank and routes them to the intake to be burned rather than released into the air.
Question 4: A P0442 code (small EVAP leak detected) is set. The FIRST step in diagnosis is:
- Replace the purge solenoid
- Replace the fuel cap
- Perform a smoke test to locate the leak source (Correct answer)
- Replace the EVAP canister
Correct answer: Perform a smoke test to locate the leak source
A smoke test is the most reliable and efficient method to locate small EVAP system leaks before replacing parts.
Question 5: An oxygen sensor that switches slowly between rich and lean is described as:
- Biased rich
- Lazy or degraded (Correct answer)
- Normal for a warm sensor
- Cross-counting correctly
Correct answer: Lazy or degraded
A slow-switching O2 sensor is considered lazy or degraded, meaning it no longer responds quickly enough to support accurate closed-loop fuel control.
Question 6: During a catalyst monitor drive cycle, the downstream O2 sensor switches as frequently as the upstream sensor. This indicates:
- The catalyst is functioning efficiently
- The catalyst efficiency is low and it is failing (Correct answer)
- The upstream O2 sensor is faulty
- The engine is running lean
Correct answer: The catalyst efficiency is low and it is failing
A healthy catalyst dampens exhaust gas composition swings, so the downstream sensor should switch far less frequently than the upstream sensor.
ASE4 Emission Control Systems 1
The EGR (Exhaust Gas Recirculation) system primarily reduces which exhaust emission?