DFFT Exhaust and Emissions 2

Question 1

A P0420 code (catalyst efficiency below threshold) is stored. Before replacing the catalytic converter, what should be done?

Accepted Answer

Diagnose and repair any misfires, rich conditions, or O2 sensor faults that could damage or misdiagnose the converter

Answer

A P0420 can be caused by a genuinely failed converter, but it can also be caused by contaminated or improperly functioning oxygen sensors or upstream engine issues that damage the converter.

Question 2

A diesel vehicle has excessive black smoke at all operating conditions. What is the most likely cause?

Accepted Answer

Over-fueling from a faulty injector, high fuel pressure, or a boost pressure fault

Answer

Persistent black smoke on a diesel means excessive unburned fuel at all loads, which is caused by over-fueling due to injector faults, incorrect timing, or insufficient air from a boost failure.

Question 3

A diesel particulate filter (DPF) becomes prematurely clogged. What is the most common operational cause?

Accepted Answer

Frequent short trips that prevent the DPF from reaching regeneration temperature

Answer

DPF regeneration requires sustained exhaust temperatures above approximately 600°C, which is only achieved on longer drives; constant short trips allow soot to accumulate without burning off.

Question 4

What does a wideband (UEGO) oxygen sensor measure that a traditional narrowband sensor cannot?

Accepted Answer

The actual air-fuel ratio across a wide range, not just rich or lean

Answer

A wideband sensor produces a linear voltage or current output proportional to the actual lambda value, enabling the ECU to precisely target any air-fuel ratio rather than just toggling around stoichiometric.

Question 5

Blue-gray smoke from the exhaust at all operating temperatures indicates what condition?

Accepted Answer

Engine oil being burned in the combustion chambers

Answer

Blue smoke is burning oil; it can be caused by worn valve stem seals, worn piston rings, or a failed PCV system allowing oil vapor to enter the intake.

DFFT - Driver Fault Finding Test Practice Test

DFFT - Driver Fault Finding Test Practice Test

DFFT Exhaust and Emissions 2