Emission Control Systems Flashcards

A P0401 code (insufficient EGR flow) is stored. Before replacing the EGR valve, the technician should check:

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.

The AIR (Secondary Air Injection) system is MOST active:

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.

Which emission is directly measured by the upstream (pre-catalyst) oxygen sensor?

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.

A vehicle fails an OBD-II plug-in emissions test. The technician is told two monitors are 'not ready.' The BEST approach is to:

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.

Excess hydrocarbons (HC) in the exhaust are MOST commonly caused by:

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.

A scan tool shows the EVAP purge solenoid is commanded ON but the fuel trims show no change. This indicates:

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.