ASE L1 powertrain controls and OBD II practice questions.

Five free practice questions on powertrain controls and OBD II: content area B of the ASE L1 Advanced Engine Performance Specialist test, 16 of its 50 scored questions (32%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Spec reading · Computerized Powertrain Controls Diagnosis (including OBD II)

   At sea level, a warm engine idles rough with no DTCs. The scan tool shows BARO 29.9 in.Hg and MAP 18.5 in.Hg (absolute), and a vacuum gauge on the intake manifold reads 11.5 in.Hg. The manufacturer specifies 18 to 20 in.Hg of vacuum at idle. What do these readings show?

   1. The MAP sensor is right; vacuum is truly low
   2. The MAP sensor reads high; vacuum is normal
   3. The MAP sensor reads low; vacuum is too high
   4. Vacuum is within the specified range
   Show the answer

   A. Correct. Manifold vacuum is BARO minus MAP: 29.9 - 18.5 = 11.4 in.Hg, which agrees with the gauge's 11.5 in.Hg, so the sensor is reporting accurately. Both are well below the 18 to 20 in.Hg specification, so the engine really is pulling weak vacuum and the next tests are mechanical (valve timing, a vacuum leak, an exhaust restriction).

   Why not the others

   B. A MAP sensor reading high would show less vacuum than the gauge. Here the scan figures work out to 11.4 in.Hg (29.9 - 18.5), matching the gauge, and both fall below the 18 in.Hg minimum.

   C. A sensor reading low would show more vacuum than really exists. The calculated 11.4 in.Hg matches the gauge's 11.5 in.Hg, and both are low, not high.

   D. The MAP figure of 18.5 in.Hg falls inside 18 to 20 in.Hg only if it is misread as vacuum; it is absolute pressure. Vacuum is 29.9 - 18.5 = 11.4 in.Hg, matching the gauge's 11.5 in.Hg and well below the specification.

2. Diagnosis · Computerized Powertrain Controls Diagnosis (including OBD II)

   A sedan's battery goes dead overnight, and its key-off draw stays at 0.9 amp; the manufacturer allows 50 mA once the modules sleep, which the service information says takes up to 20 minutes. Forty minutes after the doors are locked, a lab scope at the data link connector (DLC) still shows steady message traffic on the high-speed CAN bus. What do these results show?

   1. A shorted terminating resistor
   2. A module keeping the network awake
   3. A battery that cannot hold a charge
   4. CAN low shorted to ground
   Show the answer

   B. Correct. Modules exchange messages only while they are awake, so steady bus traffic 40 minutes after locking, twice the stated sleep time, means something is keeping the network awake, and the awake modules explain the 0.9-amp draw. The next step is finding which module keeps sending or keeps waking the others, before any battery is replaced.

   Why not the others

   A. A shorted terminating resistor would pull the two bus lines together and stop readable messages; it would not keep traffic running after the modules should sleep.

   C. The battery is the victim here: a 0.9-amp draw, far above the 50 mA allowed, would flatten a good battery overnight. A weak battery would not keep messages running on the bus.

   D. A CAN line shorted to ground stops the bus from carrying readable messages. The scope shows steady traffic, so the bus wiring is working.

3. Tech ab · Computerized Powertrain Controls Diagnosis (including OBD II)

   A late-model pickup hesitates on light acceleration, no DTCs are stored, and the technician has heard of the same complaint on other trucks of this model. Technician A says the manufacturer's technical service bulletins for this vehicle should be searched before testing begins. Technician B says that with no DTCs stored, the place to start in the service information is its symptom diagnosis for hesitation, not a DTC chart. Who is right?

   1. A only
   2. B only
   3. Both A and B
   4. Neither A nor B
   Show the answer

   C. Correct. Both technicians are right. Technician A's bulletin search can turn up a known fix before testing time is spent, and Technician B is right that with no DTC stored, the service information's symptom diagnosis for hesitation, not a DTC chart, lays out the tests for this complaint.

   Why not the others

   A. Technician B is right as well: with no DTC to follow, the manufacturer's symptom-based diagnosis for hesitation is the procedure that applies. Technician A is right that bulletins come first for a known pattern complaint.

   B. Technician A is right too: a pattern complaint on a late-model vehicle is exactly what a technical service bulletin may already cover with a revised repair. Technician B is right that a no-DTC complaint starts from the symptom diagnosis.

   D. Both are right. Checking bulletins first (Technician A) and starting from the symptom diagnosis when no DTC is stored (Technician B) are both sound ways to locate the right information before testing.

4. Tech ab · Computerized Powertrain Controls Diagnosis (including OBD II)

   A technician is about to check an engine coolant temperature (ECT) sensor and its wiring with an ohmmeter. Technician A says the circuit being measured must be unpowered, since the ohmmeter works by passing its own small current through it. Technician B says the sensor's reading means something only when compared with the manufacturer's resistance chart at the sensor's actual temperature. Who is right?

   1. A only
   2. B only
   3. Both A and B
   4. Neither A nor B
   Show the answer

   C. Correct. Technician A: an ohmmeter calculates resistance from the small current it supplies, so any voltage in the circuit makes the reading meaningless and can harm the meter. Technician B: an ECT sensor is a thermistor whose resistance changes with temperature, so a reading is compared with the chart value for the coolant temperature at that moment.

   Why not the others

   A. Not Technician A alone: Technician B is right too. Technician A is correct that an ohmmeter reads resistance from its own test current, so circuit voltage spoils the reading; Technician B is correct that an ECT thermistor's resistance changes with temperature, so it is judged against the chart at the temperature it is actually at.

   B. Not Technician B alone: Technician A is right too, since the ohmmeter measures by sending its own current, so the circuit must be unpowered (key off or the connector unplugged) for a valid reading. Technician B is right that a thermistor's value is checked against the chart at its actual temperature.

   D. Both technicians are right. Technician A: resistance is measured with the circuit unpowered, because the meter uses its own current. Technician B: a thermistor reading means something only against the chart value at the sensor's actual temperature.

5. Tech ab · Computerized Powertrain Controls Diagnosis (including OBD II)

   A port-injected V6 with sequential injection stumbles now and then at light cruise, with no DTC. All six injectors share one fused feed wire. Technician A says a low-amp current probe on that feed is no help, because the six injector currents add up to one steady reading. Technician B says the probe on the feed, connected to a lab scope, shows each injector's current pulse in turn, so a dropped pulse would show as a gap. Who is right?

   1. A only
   2. B only
   3. Both A and B
   4. Neither A nor B
   Show the answer

   B. Correct. Technician A is wrong: a sequential system opens one injector at a time, and at light cruise each pulse ends long before the next begins, so the shared feed's current is a row of separate pulses. Technician B is right: a current probe on that feed shows every injector's pulse in firing order on one trace, so an injector that drops out leaves a gap, caught without unplugging anything.

   Why not the others

   A. Only Technician B is right. Technician A is wrong: with sequential injection the injectors open one after another, so the shared feed carries a row of separate current pulses, not one steady sum. Technician B is right that the scope shows each pulse in turn, so a missed one leaves a gap.

   C. Technician A is not right. The injectors on a sequential system open one after another, so their currents do not merge into a steady reading on the shared feed. Technician B is right that the probe shows each injector's pulse in turn and a dropped pulse as a gap.

   D. Technician B is right: a current probe on the shared feed shows each injector's current pulse in turn, so one that drops out leaves a gap in the row. Technician A is wrong, because sequential injectors open one at a time and do not add up to one steady current.

02Answer key

03What area B covers

This content area covers the powertrain computer and OBD II: software versions and reprogramming, when DTCs set and clear and when the MIL comes on, data stream, freeze frame, monitors, readiness and drive cycles, network and immobilizer faults, voltage drop, current and resistance tests, sensor and actuator tests by scan data or waveform, and measuring directly to confirm what the scan tool shows.

The ASE test specification calls this content area “Computerized Powertrain Controls Diagnosis (including OBD II)”. It carries 16 of the 50 scored questions, 32% of the test.

• Check software versions and reprogram the PCM
• Research when DTCs set and clear and the MIL lights
• Read data stream, freeze frame, monitors and readiness
• Separate control problems from mechanical ones
• Diagnose data network and immobilizer faults
• Run voltage drop, current and resistance tests
• Test sensors and actuators with scan data and waveforms
• Measure directly to confirm scan data and unseen circuits

04What the questions turn on

A command is not a result

A command PID reports what the PCM tells its driver to do, not what happens after. A fuel pump relay PID reading ON does not prove the pump runs. Injector pulse width of 0.0 ms is the PCM choosing not to fuel.

Confirm scan data directly

A scan value is checked by measuring the same signal directly. When a sensor makes voltage at its connector but the scan tool shows none, the signal is lost on the way. When BARO minus MAP matches a vacuum gauge, the MAP sensor is accurate.

Pending, confirmed and the MIL

The composite vehicle questions apply the reference booklet's DTC rules. A circuit fault caught by the comprehensive component monitor confirms and lights the MIL on the first trip. A system monitor such as EGR stores a pending DTC first and confirms on a repeat failure.

Which modules still answer

An open data wire to one module cuts off only that module, so the scan tool still reaches the others. A data line shorted to ground stops the whole bus. A line resting at 0 volts instead of its normal level is held at ground.

05The other content areas

The L1 test has six content areas. Each has its own page of free questions:

The ASE L1 Advanced Engine Performance Specialist practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.

ToolboxPrep is an independent study site. It is not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original practice questions written to the published ASE test specifications; they are not actual test questions.