SCT OBD Systems & Diagnostics 2 — Questions and Answers
Question 1: What DTC prefix indicates a generic (SAE-standardized) emissions-related powertrain fault in the OBD II system?
- B0 (Body)
- C0 (Chassis)
- P0 (Generic Powertrain) (Correct answer)
- U0 (Network Communication)
Correct answer: P0 (Generic Powertrain)
P0 codes are generic SAE-standardized powertrain diagnostic trouble codes, and most emissions-related faults are classified under this category.
Question 2: What does a 'pending' DTC in an OBD II system indicate?
- A fault detected once but not yet confirmed through a second drive cycle (Correct answer)
- A fault that has been confirmed and stored as a permanent code
- A code that only exists when the MIL is continuously illuminated
- A historical code stored from a previous model year inspection
Correct answer: A fault detected once but not yet confirmed through a second drive cycle
A pending DTC means the OBD II system detected a potential fault during one drive cycle but requires confirmation in a second drive cycle before setting a confirmed code and illuminating the MIL.
Question 3: What engine parameter does OBD II standard PID $05 (Mode 01, PID 05) report?
- Engine RPM
- Vehicle speed
- Engine coolant temperature (Correct answer)
- Throttle position percentage
Correct answer: Engine coolant temperature
OBD II PID $05 reports engine coolant temperature, which is critical for verifying the engine has reached normal operating temperature before accurate emission tests can be performed.
Question 4: How many consecutive confirmed detections (drive cycles) does an OBD II system typically require before setting a confirmed DTC and illuminating the MIL?
- One drive cycle
- Two consecutive drive cycles (Correct answer)
- Five drive cycles
- Ten drive cycles
Correct answer: Two consecutive drive cycles
Most OBD II monitors require a fault to be detected in two consecutive drive cycles before setting a confirmed DTC and illuminating the MIL, helping prevent false positives from transient conditions.
Question 5: What does the OBD II oxygen sensor monitor evaluate during its self-test?
- The physical insulation condition of the oxygen sensor wiring harness
- The response time and output voltage range of upstream and downstream O2 sensors (Correct answer)
- Whether the oxygen sensors are the correct OEM part number for the vehicle
- The oxygen content of ambient air entering the engine air filter
Correct answer: The response time and output voltage range of upstream and downstream O2 sensors
The oxygen sensor monitor evaluates the switching frequency, response time, and voltage output of both upstream (pre-cat) and downstream (post-cat) O2 sensors to verify proper fuel control feedback.
Question 6: Which OBD II communication protocol became mandatory for all light-duty vehicles sold in the United States starting with model year 2008?
- ISO 9141-2
- SAE J1850 VPW
- ISO 15765-4 (CAN) (Correct answer)
- SAE J1850 PWM
Correct answer: ISO 15765-4 (CAN)
ISO 15765-4 (Controller Area Network/CAN) became the mandatory OBD II communication protocol for all light-duty US vehicles beginning with model year 2008, replacing older protocols.
Question 7: What action must a smog technician take when a vehicle arrives for inspection with a flashing (blinking) MIL?
- Proceed with the test but note the flashing light in the inspection report
- Immediately fail the vehicle, as a flashing MIL indicates active catalyst-damaging misfires (Correct answer)
- Reset the MIL with a scan tool and retest to see if it illuminates again
- Only fail the vehicle if P0300-range misfire codes are also stored
Correct answer: Immediately fail the vehicle, as a flashing MIL indicates active catalyst-damaging misfires
A flashing MIL indicates active severe misfires that can damage the catalytic converter; the vehicle must be immediately failed and the owner directed to seek repair before testing.
SCT OBD Systems & Diagnostics 2
What DTC prefix indicates a generic (SAE-standardized) emissions-related powertrain fault in the OBD II system?