ASE Fuel Systems & Emissions Control 2

Question 1

What does a three-way catalytic converter simultaneously reduce?

Accepted Answer

HC, CO, and NOx emissions

Answer

A three-way catalyst uses oxidation (platinum/palladium) to convert HC and CO into CO2 and H2O, while a reduction catalyst (rhodium) breaks NOx down into harmless N2 and O2.

Question 2

A Long Term Fuel Trim (LTFT) reading of +18% most likely indicates:

Accepted Answer

A lean condition — the ECM is adding fuel to compensate

Answer

Positive fuel trim means the ECM is commanding more fuel than the base calibration calls for, indicating a lean condition caused by a vacuum leak, weak fuel pump, dirty injectors, or a faulty MAF sensor.

Question 3

What is the purpose of the PCV (Positive Crankcase Ventilation) valve?

Accepted Answer

Vent blow-by gases from the crankcase into the intake manifold for combustion

Answer

The PCV valve meters crankcase blow-by gases (containing unburned hydrocarbons and combustion byproducts) into the intake manifold where they are burned, preventing their release as pollutants.

Question 4

A Long Term Fuel Trim (LTFT) reading of -16% most likely indicates:

Accepted Answer

A rich condition — the ECM is reducing fuel to compensate

Answer

Negative fuel trim means the ECM is cutting fuel delivery below the base calibration, indicating a rich condition caused by leaking injectors, excessively high fuel pressure, or a MAF sensor reading too high.

Question 5

Which OBD-II readiness monitor specifically evaluates the EVAP system for leaks?

Accepted Answer

EVAP monitor

Answer

The EVAP monitor performs sealed-system pressure and vacuum tests under specific drive-cycle conditions to detect leaks as small as 0.020 inches, setting a DTC if the system fails to hold.

ASE Practice Test

ASE Practice Test

ASE Fuel Systems & Emissions Control 2