GM Fuel Systems and Emissions 2 — Questions and Answers
Question 1: What is the role of the catalytic converter on a GM vehicle in reducing emissions?
- Increase exhaust back pressure to improve low-end torque
- Oxidize HC and CO and reduce NOx through chemical reactions using precious metal catalysts (Correct answer)
- Filter particulate matter from the exhaust stream using a ceramic wall-flow filter
- Recirculate exhaust gases back into the intake manifold
Correct 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.
Question 2: Which OBD-II monitor specifically evaluates catalytic converter efficiency on GM vehicles?
- Oxygen Sensor Monitor
- Catalyst Monitor (Correct answer)
- Comprehensive Components Monitor
- EGR System Monitor
Correct 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.
Question 3: On GM vehicles with Exhaust Gas Recirculation (EGR), what is the primary emission the EGR system is designed to reduce?
- Hydrocarbons (HC)
- Carbon monoxide (CO)
- Nitrogen oxides (NOx) (Correct answer)
- Carbon dioxide (CO2)
Correct 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.
Question 4: What does a P0420 DTC indicate on a GM vehicle?
- Upstream oxygen sensor circuit malfunction
- Catalyst system efficiency below threshold (Bank 1) (Correct answer)
- EVAP system large leak detected
- Fuel pressure sensor out of range
Correct 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.
Question 5: On a GM vehicle, what condition would cause the PCM to set a P0171 (System Too Lean, Bank 1) diagnostic trouble code?
- Fuel injector stuck open causing excessive fuel delivery
- A large vacuum leak causing unmetered air to enter the intake (Correct answer)
- A rich exhaust condition from a flooded engine
- Downstream oxygen sensor reading 0.8V continuously
Correct 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.
Question 6: What is the purpose of the Secondary Air Injection (AIR) system used on some GM engines during cold start?
- Provide additional air to the fuel injectors to lean out the mixture at idle
- Pump fresh air into the exhaust stream to oxidize unburned HC and CO during warm-up (Correct answer)
- Pressurize the fuel system above normal operating pressure for cold-start enrichment
- Supply air to the EGR valve to promote exhaust gas mixing
Correct 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.
Question 7: On GM GDI engines, what is a commonly recommended maintenance procedure to address carbon buildup on intake valves?
- Replace the fuel filter every 15,000 miles to improve atomization
- Perform an intake valve cleaning using walnut blasting or approved chemical decarbonization (Correct answer)
- Increase injector pulse width via PCM calibration to wash valves with extra fuel
- Replace the oxygen sensor to reset adaptive fuel trims and improve combustion
Correct 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.
GM Fuel Systems and Emissions 2
What is the role of the catalytic converter on a GM vehicle in reducing emissions?