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OBD II and Scan Tool Diagnostics Flashcards

7 cards from real ASE4 practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 OBD II and Scan Tool Diagnostics flashcards as text

  1. What fault condition is indicated by a P0300 diagnostic trouble code?

    Answer: Random or multiple cylinder misfire detected

    P0300 is the generic OBD II code for a random or multiple cylinder misfire, meaning misfires are not isolated to a single identifiable cylinder.

  2. When Mode $04 is executed from a scan tool, what action does it perform?

    Answer: Clears all stored DTCs, freeze frame data, and resets readiness monitors to incomplete

    Mode $04 erases all confirmed and pending DTCs, clears associated freeze frame data, and resets all readiness monitors to 'incomplete' status, requiring a new drive cycle to re-complete them.

  3. What data does OBD II Mode $09 contain?

    Answer: Vehicle information including VIN, ECU calibration IDs, and calibration verification numbers

    Mode $09 provides vehicle identification information including the 17-character VIN, ECU calibration identification strings, and CVNs (Calibration Verification Numbers) used to confirm factory programming.

  4. A P0440 series diagnostic trouble code indicates a fault in which emission control system?

    Answer: Evaporative emission control (EVAP) system

    P0440 through P0449 codes are assigned to the evaporative emission control system, which captures fuel tank vapors and prevents them from escaping to the atmosphere.

  5. How does the OBD II catalyst efficiency monitor (P0420) evaluate catalytic converter performance?

    Answer: It compares upstream and downstream O2 sensor switching activity to evaluate oxygen storage capacity

    The catalyst monitor compares the switching frequency of the upstream (pre-cat) and downstream (post-cat) O2 sensors — a healthy converter stores and releases oxygen, causing the downstream sensor to switch less frequently than the upstream sensor.

  6. Which OBD II readiness monitor would most directly be affected by a faulty EVAP purge solenoid valve?

    Answer: Evaporative emission (EVAP) monitor

    A faulty EVAP purge solenoid prevents the system from completing its purge flow and leak detection tests, causing the EVAP readiness monitor to remain incomplete.

  7. When a scan tool shows LTFT of +22% at idle that drops to +3% at 2500 RPM, the most likely diagnosis is:

    Answer: A vacuum leak causing an idle-specific lean condition

    A vacuum leak causes the greatest lean effect at idle — where intake manifold vacuum is highest and airflow is lowest — but diminishes significantly as engine load increases; this load-dependent fuel trim pattern is the classic vacuum leak signature.