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OBD Systems & Diagnostics Flashcards

7 cards from real SCT 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. What is 'freeze frame' data captured and stored by an OBD II system?

    Answer: A snapshot of engine operating conditions recorded at the moment a DTC was set

    Freeze frame data is a snapshot of critical engine parameters (RPM, load, speed, fuel trim, coolant temp, etc.) captured at the instant a DTC was confirmed, helping technicians diagnose the root cause.

  2. Which of the following OBD II monitors is classified as a 'continuous' monitor that runs at all times while the engine is operating?

    Answer: Misfire monitor

    The misfire monitor is a continuous monitor that runs at all times while the engine operates, whereas the catalyst, EVAP, and oxygen sensor monitors are non-continuous monitors requiring specific drive conditions.

  3. For California smog inspections of 2001 and newer OBD II vehicles, what is the maximum number of readiness monitors allowed to show 'not ready' while still permitting the vehicle to pass?

    Answer: One monitor may be incomplete

    California regulations allow a maximum of one incomplete readiness monitor for 2001 and newer OBD II vehicles, provided no DTCs are stored and the MIL is extinguished.

  4. What does long-term fuel trim (LTFT) data of consistently +20% or higher indicate about engine operation?

    Answer: The engine has been running lean and the ECM is adding extra fuel to compensate

    High positive LTFT values mean the ECM is commanding significantly more fuel than expected, indicating a lean condition caused by factors such as vacuum leaks, low fuel pressure, or faulty injectors.

  5. Which OBD II service mode (Mode) is used to request current powertrain diagnostic data and view live sensor readings?

    Answer: Mode $01

    OBD II Mode $01 (Service 01) is used to request current powertrain diagnostic data, providing live readings of parameters such as engine RPM, coolant temperature, and oxygen sensor voltages.

  6. What does diagnostic trouble code P0420 indicate in an OBD II system?

    Answer: Catalytic converter efficiency below threshold — Bank 1

    P0420 indicates that the catalyst system efficiency is below the required threshold for Bank 1, meaning the catalytic converter is not adequately converting HC, CO, and NOx into less harmful emissions.

  7. What is the primary purpose of checking OBD II I/M readiness status during a California smog inspection?

    Answer: To confirm the vehicle's emission control self-tests are complete and no faults have been detected

    Checking I/M readiness status confirms the vehicle's OBD II system has completed its self-diagnostic tests and detected no emission-related faults, serving as the primary surrogate measure of emission system health in modern smog checks.