Fuel Systems and Emissions Flashcards

What is the role of the catalytic converter on a GM vehicle in reducing emissions?

Answer: Oxidize HC and CO and reduce NOx through chemical reactions using precious metal catalysts

The three-way catalytic converter uses platinum, palladium, and rhodium to oxidize hydrocarbons (HC) and carbon monoxide (CO) and reduce nitrogen oxides (NOx) simultaneously.

Which OBD-II monitor specifically evaluates catalytic converter efficiency on GM vehicles?

Answer: Catalyst Monitor

The Catalyst Monitor compares switching frequency and voltage readings between upstream and downstream oxygen sensors to determine if the converter is storing and releasing oxygen effectively.

On GM vehicles with Exhaust Gas Recirculation (EGR), what is the primary emission the EGR system is designed to reduce?

Answer: Nitrogen oxides (NOx)

EGR reduces NOx emissions by reintroducing a metered amount of exhaust gas into the intake, lowering combustion temperatures that drive nitrogen and oxygen to combine into NOx.

What does a P0420 DTC indicate on a GM vehicle?

Answer: Catalyst system efficiency below threshold (Bank 1)

P0420 indicates the PCM has detected that the Bank 1 catalytic converter's oxygen storage efficiency has dropped below the OBD-II threshold, signaling converter degradation.

On a GM vehicle, what condition would cause the PCM to set a P0171 (System Too Lean, Bank 1) diagnostic trouble code?

Answer: A large vacuum leak causing unmetered air to enter the intake

A vacuum leak allows unmetered air past the MAF sensor, causing the PCM to see a lean condition; the PCM compensates with positive fuel trim until a P0171 is set when the limit is exceeded.

What is the purpose of the Secondary Air Injection (AIR) system used on some GM engines during cold start?

Answer: Pump fresh air into the exhaust stream to oxidize unburned HC and CO during warm-up

The AIR system pumps ambient air into the exhaust manifold during cold start, providing oxygen to catalyze oxidation of HC and CO before the catalytic converter reaches operating temperature.

On GM GDI engines, what is a commonly recommended maintenance procedure to address carbon buildup on intake valves?

Answer: Perform an intake valve cleaning using walnut blasting or approved chemical decarbonization

Because GDI injectors do not spray fuel over the intake valves, oil vapor from the PCV system deposits carbon on the valves; walnut blasting or chemical cleaning restores airflow.