DFFT Exhaust and Emissions

Question 1

A vehicle fails an emissions test for high HC (hydrocarbon) levels. What is the most likely cause?

Accepted Answer

Engine misfires causing unburned fuel to pass through the exhaust

Answer

A clogged EGR valve causing high NOx production

Answer

A rich fuel mixture causing excessive CO output

Answer

A leaking exhaust manifold gasket

Question 2

A catalytic converter is found to be glowing red hot during operation. What is the most likely cause?

Accepted Answer

Unburned fuel from engine misfires is igniting inside the converter

Answer

The converter has reached normal light-off temperature

Answer

Excessive exhaust back pressure from a blocked muffler

Answer

A lean fuel mixture producing high exhaust temperatures

Question 3

A vehicle fails an emissions test for high CO (carbon monoxide). What does elevated CO indicate?

Accepted Answer

Incomplete combustion due to a rich air-fuel mixture

Answer

Excess air in the mixture causing a lean burn condition

Answer

A faulty catalytic converter substrate

Answer

A clogged diesel particulate filter

Question 4

A vehicle fails emissions for high NOx. What driving condition produces the most NOx?

Accepted Answer

High engine load at high temperatures with a lean mixture

Answer

Low-speed light-load city driving with a rich mixture

Answer

Cold engine operation before reaching operating temperature

Answer

Idle with a fully warmed engine and closed throttle

Question 5

What is the primary function of the EGR (Exhaust Gas Recirculation) valve?

Accepted Answer

To recirculate a portion of exhaust gas into the intake to lower peak combustion temperatures and reduce NOx

Answer

To route exhaust gases through the catalytic converter for additional treatment

Answer

To prevent exhaust backflow into the intake manifold under boost conditions

Answer

To recover exhaust heat for cabin heating purposes

Question 6

An oxygen sensor waveform shows a signal that is stuck at 0.2V and does not oscillate. What does this indicate?

Accepted Answer

The sensor is reading a continuously lean mixture, possibly due to a vacuum leak or failed sensor

Answer

The engine is running perfectly stoichiometric

Answer

The catalytic converter is absorbing all oxygen before the sensor

Answer

The EGR valve is stuck open diluting the exhaust