DFFT Exhaust and Emissions 1 — Questions and Answers
Question 1: A vehicle fails an emissions test for high HC (hydrocarbon) levels. What is the most likely cause?
- Engine misfires causing unburned fuel to pass through the exhaust (Correct answer)
- A clogged EGR valve causing high NOx production
- A rich fuel mixture causing excessive CO output
- A leaking exhaust manifold gasket
Correct answer: Engine misfires causing unburned fuel to pass through the exhaust
HC emissions are unburned fuel; engine misfires cause raw fuel to exit the combustion chamber unburned and pass through the catalytic converter, dramatically increasing HC output.
Question 2: A catalytic converter is found to be glowing red hot during operation. What is the most likely cause?
- Unburned fuel from engine misfires is igniting inside the converter (Correct answer)
- The converter has reached normal light-off temperature
- Excessive exhaust back pressure from a blocked muffler
- A lean fuel mixture producing high exhaust temperatures
Correct answer: Unburned fuel from engine misfires is igniting inside the converter
Large quantities of unburned HC entering the converter ignite inside it, generating extreme heat that can melt the internal substrate and destroy the converter.
Question 3: A vehicle fails an emissions test for high CO (carbon monoxide). What does elevated CO indicate?
- Incomplete combustion due to a rich air-fuel mixture (Correct answer)
- Excess air in the mixture causing a lean burn condition
- A faulty catalytic converter substrate
- A clogged diesel particulate filter
Correct answer: Incomplete combustion due to a rich air-fuel mixture
CO is produced when carbon burns without sufficient oxygen; a rich mixture means there is more fuel than available oxygen, resulting in incomplete combustion and high CO output.
Question 4: A vehicle fails emissions for high NOx. What driving condition produces the most NOx?
- High engine load at high temperatures with a lean mixture (Correct answer)
- Low-speed light-load city driving with a rich mixture
- Cold engine operation before reaching operating temperature
- Idle with a fully warmed engine and closed throttle
Correct answer: High engine load at high temperatures with a lean mixture
NOx forms when combustion temperatures exceed approximately 2500°F (1370°C), conditions that occur under high load with lean combustion promoting very high peak temperatures.
Question 5: What is the primary function of the EGR (Exhaust Gas Recirculation) valve?
- To recirculate a portion of exhaust gas into the intake to lower peak combustion temperatures and reduce NOx (Correct answer)
- To route exhaust gases through the catalytic converter for additional treatment
- To prevent exhaust backflow into the intake manifold under boost conditions
- To recover exhaust heat for cabin heating purposes
Correct answer: To recirculate a portion of exhaust gas into the intake to lower peak combustion temperatures and reduce NOx
Recirculated exhaust gas dilutes the intake charge with inert gas, lowering peak combustion temperature and reducing the formation of nitrogen oxides.
Question 6: An oxygen sensor waveform shows a signal that is stuck at 0.2V and does not oscillate. What does this indicate?
- The sensor is reading a continuously lean mixture, possibly due to a vacuum leak or failed sensor (Correct answer)
- The engine is running perfectly stoichiometric
- The catalytic converter is absorbing all oxygen before the sensor
- The EGR valve is stuck open diluting the exhaust
Correct answer: The sensor is reading a continuously lean mixture, possibly due to a vacuum leak or failed sensor
A healthy narrowband O2 sensor should oscillate between approximately 0.1V and 0.9V; a stuck low signal means either the mixture is genuinely lean or the sensor itself has failed.
DFFT Exhaust and Emissions 1
A vehicle fails an emissions test for high HC (hydrocarbon) levels.
What is the most likely cause?