ASE4 Emission Control Systems 2 — Questions and Answers
Question 1: A P0401 code (insufficient EGR flow) is stored. Before replacing the EGR valve, the technician should check:
- The catalytic converter efficiency
- The EGR passages for carbon buildup restricting flow (Correct answer)
- The oxygen sensor switching speed
- The fuel pressure regulator
Correct answer: The EGR passages for carbon buildup restricting flow
EGR passages and ports frequently become clogged with carbon deposits, causing insufficient flow without the EGR valve itself being faulty.
Question 2: The AIR (Secondary Air Injection) system is MOST active:
- At highway cruise speeds
- Immediately after cold engine startup (Correct answer)
- During wide-open throttle acceleration
- When the EGR valve is open
Correct answer: Immediately after cold engine startup
The AIR system pumps fresh air into the exhaust stream right after cold startup to help oxidize unburned HC and CO and rapidly heat the catalyst.
Question 3: Which emission is directly measured by the upstream (pre-catalyst) oxygen sensor?
- NOx concentration
- Oxygen content in the exhaust stream (Correct answer)
- Hydrocarbon content
- Carbon monoxide levels
Correct answer: Oxygen content in the exhaust stream
The upstream O2 sensor measures the oxygen content of exhaust gases, which the ECM uses to calculate rich/lean condition and adjust fuel delivery.
Question 4: A vehicle fails an OBD-II plug-in emissions test. The technician is told two monitors are 'not ready.' The BEST approach is to:
- Replace the catalytic converter and oxygen sensors
- Perform the appropriate OBD-II drive cycles to set the incomplete monitors (Correct answer)
- Clear all DTCs and retest immediately
- Inspect and replace the EGR valve
Correct answer: Perform the appropriate OBD-II drive cycles to set the incomplete monitors
Incomplete monitors must be set by performing the specific drive cycles that allow the ECM to run those self-tests.
Question 5: Excess hydrocarbons (HC) in the exhaust are MOST commonly caused by:
- High combustion temperatures
- Incomplete combustion from misfires or an overly rich mixture (Correct answer)
- Advanced ignition timing
- High EGR flow rates
Correct answer: Incomplete combustion from misfires or an overly rich mixture
Unburned fuel in the exhaust is the primary source of HC emissions, typically caused by misfires, a rich mixture, or poor combustion.
Question 6: A scan tool shows the EVAP purge solenoid is commanded ON but the fuel trims show no change. This indicates:
- The EVAP canister is fully purged and working normally
- The purge solenoid is not opening or the canister is empty/disconnected (Correct answer)
- A vacuum leak downstream of the purge solenoid
- The EVAP system is working correctly
Correct answer: The purge solenoid is not opening or the canister is empty/disconnected
When the purge solenoid opens, vapors should enrich the mixture and cause the ECM to subtract fuel — no change means no vapors are flowing.
ASE4 Emission Control Systems 2
A P0401 code (insufficient EGR flow) is stored.
Before replacing the EGR valve, the technician should check: