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Exhaust and Emissions Flashcards

6 cards from real DFFT practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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

    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.