ASE4 OBD II and Scan Tool Diagnostics 2 — Questions and Answers
Question 1: What type of information does OBD II Mode $06 provide to a technician?
- A list of all current and pending DTCs with descriptions
- On-board monitoring test results showing actual values versus pass/fail thresholds (Correct answer)
- Live PID data stream from all continuously monitored sensors
- Freeze frame data from the most recently stored fault code
Correct answer: On-board monitoring test results showing actual values versus pass/fail thresholds
Mode $06 reports the results of non-continuously monitored system tests, displaying actual measured values alongside minimum and maximum thresholds — useful for identifying borderline failures before a DTC sets.
Question 2: A 'pending' DTC stored in Mode $07 means:
- The DTC is confirmed and the MIL has been illuminated
- A fault was detected on one trip but has not yet been confirmed on a second trip (Correct answer)
- The DTC was previously cleared but the original fault is still causing problems
- The fault is only detected under extreme hot or cold temperature conditions
Correct answer: A fault was detected on one trip but has not yet been confirmed on a second trip
A pending DTC indicates the OBD II system detected a fault during one trip but requires a second trip confirmation before illuminating the MIL, based on the two-trip detection strategy.
Question 3: On a scan tool, OBD II Mode $01 PID $0C displays which parameter?
- Vehicle speed in mph
- Engine coolant temperature in degrees Fahrenheit
- Engine RPM calculated from the crankshaft position sensor (Correct answer)
- Throttle position sensor voltage
Correct answer: Engine RPM calculated from the crankshaft position sensor
PID $0C in Mode $01 (current data) reports calculated engine RPM derived from the crankshaft position sensor signal frequency.
Question 4: A short-term fuel trim (STFT) reading of +15% indicates the ECM is:
- Reducing fuel delivery because the engine is running rich
- Adding fuel to compensate for a lean condition detected by the O2 sensor (Correct answer)
- Disabling one fuel injector to balance the air/fuel mixture
- Advancing ignition timing to improve combustion efficiency
Correct answer: Adding fuel to compensate for a lean condition detected by the O2 sensor
A positive STFT means the ECM is commanding more fuel (15% above the base fueling map) in real-time response to an oxygen sensor indicating a lean exhaust condition.
Question 5: What is the primary function of long-term fuel trim (LTFT) stored in the ECM?
- To provide immediate fuel delivery corrections based on real-time O2 sensor feedback
- To store learned baseline fuel corrections that persist across ignition cycles (Correct answer)
- To monitor catalytic converter oxygen storage capacity over time
- To control EGR valve duty cycle during warm-up enrichment
Correct answer: To store learned baseline fuel corrections that persist across ignition cycles
LTFT stores accumulated fuel delivery corrections in non-volatile ECM memory, compensating long-term for changes such as injector wear, vacuum leaks, sensor drift, or fuel pressure variations.
Question 6: If both STFT and LTFT are consistently at -15% across all load conditions, the most likely cause is:
- A large vacuum leak causing excessive unmetered air entry
- Excessive fuel delivery from high fuel pressure or leaking injectors (Correct answer)
- A faulty upstream O2 sensor stuck in lean indication
- An EGR valve stuck in the closed position
Correct answer: Excessive fuel delivery from high fuel pressure or leaking injectors
Consistently negative fuel trims indicate a persistent rich condition — the ECM is reducing fuel significantly, pointing to causes such as excessive fuel pressure, leaking injectors, or a purged evaporative canister flooding the intake.
Question 7: Which pin of the OBD II DLC (Data Link Connector) provides unswitched battery voltage to power the scan tool?
- Pin 2
- Pin 4
- Pin 16 (Correct answer)
- Pin 14
Correct answer: Pin 16
Pin 16 of the standardized 16-pin OBD II DLC provides constant (unswitched) battery power so the scan tool remains powered regardless of ignition switch position.
ASE4 OBD II and Scan Tool Diagnostics 2
What type of information does OBD II Mode $06 provide to a technician?