Ten ASE L1 Advanced Engine Performance Specialist questions in the ASE format: four options, one best answer. They are free to use, print and share, and there is nothing to sign up for on this page.
01How to use these
The questions come from the ToolboxPrep L1 bank and are spread across the test's content areas in the proportion the ASE test specification gives them: A (General Powertrain Diagnosis), B (Computerized Powertrain Controls Diagnosis (including OBD II)), C (Ignition System Diagnosis), D (Fuel Systems and Air Induction Systems Diagnosis), E (Emission Control Systems Diagnosis), F (I/M Failure Diagnosis). Answer each one before opening the explanation. Every explanation covers all four options, because knowing why the wrong answers are wrong is most of the skill.
- Pace. The L1 test gives you 150 minutes for 60 questions (50 scored, 10 unscored), about 2 min 30 s a question. Try these at that pace.
- Print. The printed version shows the questions and the answer key without the explanations, so it works as a handout.
02The questions
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On a port-injected engine with heavy carbon deposits on the backs of the intake valves, why does the engine hesitate mostly during the first minutes after a cold start?
- Cold deposits release extra fuel, making the mixture rich
- The deposits block more air while the valves are cold
- The deposits raise compression and cause spark knock
- The deposits soak up injected fuel, leaning the mixture
Show the answer
D. Correct. Porous carbon on the valves absorbs fuel sprayed at them until it is saturated, so a cold engine, which already struggles to vaporize fuel, runs lean and hesitates. Once the deposits are soaked and the engine is warm, the mixture recovers, which is why the complaint fades.
Why not the others
A. This has it backwards: cold deposits take fuel out of the charge as they soak it up, leaning the mixture. They give fuel back later, once saturated and warm.
B. Carbon does not swell enough with temperature to block more air when cold, and a small airflow restriction would simply be measured and fueled for by the PCM rather than cause a cold-only lean hesitation. The deposits act like a sponge for fuel.
C. Combustion chamber deposits can raise compression, but knock shows up hot and under load, not in the first minutes after a cold start. Valve deposits soaking up fuel explain a cold-only hesitation.
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Before judging whether a warm engine's fuel trims show a real mixture problem, which scan-tool reading should the technician confirm first?
- The upstream oxygen sensor signal
- Fuel system loop status
- Engine coolant temperature
- Calculated load value
Show the answer
B. Correct. Short-term trim corrects the mixture from oxygen sensor feedback only while the PCM is in closed loop; in open loop it stops responding to the sensor, and the trims then say little about the mixture. Confirming closed loop first makes the trim readings worth judging.
Why not the others
A. An oxygen sensor can switch normally while the PCM ignores it in open loop, so its voltage alone does not show whether the trims are being driven by sensor feedback. Loop status does.
C. A warm coolant reading is one of the conditions many PCMs need before closed loop, but it is not the only one. Loop status reports whether the PCM has actually closed the loop.
D. Load tells the technician which operating range the trims were read in, which matters once feedback is confirmed, but it does not show whether the PCM is using sensor feedback at all.
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A port-injected engine with a mass airflow (MAF) sensor pings (spark knock) under load on fuel of the octane rating the manufacturer specifies. Any of these control-system faults could cause this EXCEPT:
- an upstream HO2S stuck reading lean.
- a MAF sensor that reports too little airflow.
- an EGR valve that does not open when commanded.
- an IAT sensor that reads colder than actual.
Show the answer
A. Correct. A sensor that always reads lean makes the PCM add fuel, so the engine runs rich, and the extra fuel cools the charge and resists knock. The other three faults lean the mixture, add spark advance or raise combustion temperature, each of which can cause spark knock.
Why not the others
B. Too little measured airflow makes the PCM calculate a lighter load, so it uses less fuel and a more advanced spark value than the real load needs; both raise knock tendency under load.
C. Recirculated exhaust dilutes the charge and lowers combustion temperature; when the valve stays shut under load, combustion runs hotter and the engine can knock.
D. PCMs typically pull spark advance back as intake air gets hotter. An IAT reading colder than the real air leaves too much advance for the hot charge, which can cause knock under load.
-
A low-amp current probe on an in-tank fuel pump's feed wire is connected to a lab scope. The current trace shows a regular row of small humps, but one hump is much lower than the rest and the low hump repeats once every revolution of the pump motor. What does this pattern indicate?
- A restricted fuel filter downstream of the pump
- A bad commutator segment in the pump motor
- Low supply voltage at the pump's feed terminal
- A clogged strainer on the pump's inlet side
Show the answer
B. Correct. Each hump is the current pulse as the brushes pass from one commutator segment and its armature winding to the next; a segment or winding that is worn, open or high in resistance passes less current, so the same low hump returns every revolution. The pump motor is failing even if pressure is still in specification today.
Why not the others
A. A restriction makes the pump work harder and raises its average current, but it changes every hump the same way. It cannot make one hump low once per revolution.
C. Low feed voltage lowers the current through every segment equally and slows the pump; the humps stay even. A single low hump that repeats each revolution comes from inside the motor.
D. A clogged inlet changes the pump's load, which shifts all of the humps together. A low hump that repeats once per revolution is a fault in the motor's armature or commutator.
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After a previous shop's tune-up, an engine's new spark plugs carbon-foul within 3,000 miles of mostly highway driving, though coolant temperature and fuel trims are normal. The technician first wants to confirm that the installed plugs are the heat range the engine's manufacturer specifies. Which check does this?
- Measure each plug's gap against the specified gap
- Compare insulator tip color from cylinder to cylinder
- Compare each plug's thread reach with the specified reach
- Check the part number against the specified plug
Show the answer
D. Correct. Heat range is built into the plug's design, mainly the length of the insulator nose and so of its heat path to the shell, and no gauge or meter can measure it; it is identified by the plug maker's part number. Checking that number against the plug the manufacturer specifies (through the plug maker's cross-reference for another brand) shows whether a colder plug was installed.
Why not the others
A. A plug of any heat range can be gapped to the specification, so a correct gap says nothing about heat range; heat range is identified by the plug's part number, checked against the specified plug.
B. If every plug is the same wrong heat range, every tip can look alike, so comparing cylinders cannot show it; tip color shows how the plugs ran, while the part number shows what heat range they are.
C. Reach is the threaded length that sets how far the plug enters the chamber; plugs of the same reach come in different heat ranges, so a matching reach cannot confirm heat range; the part number identifies it.
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A gasoline direct injection (GDI) engine set P0300 “Random/Multiple Cylinder Misfire Detected” only during the first minute after cold starts, and heavy carbon deposits on its intake valves were cleaned off. Which check best verifies the repair?
- Misfire counts on an overnight cold start
- Misfire counts at warm idle just after the cleaning
- The MIL going out after the DTC is cleared
- Cranking compression checked on each cylinder
Show the answer
A. Correct. The misfire happened only in the first minute after a cold start, so the repair is proven only by recreating that condition: a cold start after the engine has sat overnight, watching the misfire counters. Normal counts then show the deposits were the cause and are gone.
Why not the others
B. The engine never misfired warm, so zero misfire counts at warm idle say nothing about the repair. The check has to repeat the original condition, a cold start after an overnight soak.
C. Clearing the DTC turns the MIL off whether or not the fault is fixed, so this proves nothing. Watching the misfire counts on a cold start after an overnight soak does.
D. Compression shows ring and valve sealing, not whether the cold-start misfire is gone. The repair is verified by watching misfire counts on a cold start after an overnight soak.
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On an engine with air/fuel ratio sensors, what does the generic OBD II commanded equivalence ratio PID show?
- The fuel split between the two banks
- The lambda the sensor is measuring
- The ratio of short- to long-term trim
- The lambda the ECM is aiming for
Show the answer
D. Correct. Commanded equivalence ratio is the air/fuel ratio target, in lambda, that the ECM is trying to deliver: near 1.00 in normal closed-loop running and below 1.00 when it enriches, such as at full load. Comparing it with the measured lambda from the air/fuel ratio sensor shows whether fuel control is reaching its command.
Why not the others
A. Each bank's mixture is controlled and reported through its own sensor and trims. The commanded equivalence ratio is the ECM's lambda target, not a comparison between banks.
B. The measured value comes from the air/fuel ratio sensor's own PID. The commanded equivalence ratio is the ECM's target, so comparing the two shows whether fuel control is achieving it.
C. Fuel trims are reported as separate percentages for short-term and long-term correction; no PID divides one by the other. The commanded equivalence ratio is the lambda the ECM is targeting.
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A three-way catalytic converter on a warm engine with normal fuel control has lost most of its efficiency. Compared with its readings when the converter was new, the tailpipe reading for any of these gases will rise EXCEPT:
- hydrocarbons (HC).
- carbon monoxide (CO).
- oxygen (O2).
- carbon dioxide (CO2).
Show the answer
D. Correct. CO2 is the product of the converter's oxidation of HC and CO, so when the converter stops working less CO2 is made and the reading drops slightly, while HC, CO and the unused O2 rise. NOx rises too, because the reduction side has failed as well.
Why not the others
A. HC that the converter used to oxidize now passes through, so tailpipe HC rises. It could rise; the gas that falls is CO2.
B. A working converter oxidizes CO to CO2; a failed one lets it out, so tailpipe CO rises. The reading that drops is CO2.
C. A working converter uses exhaust oxygen to oxidize HC and CO; when it stops working that oxygen passes through, so tailpipe O2 rises. It could rise; the gas that falls is CO2.
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An EGR system on a warm engine stops flowing when the PCM commands it at light cruise. Any of these could result EXCEPT:
- spark knock on acceleration.
- a NOx failure on a loaded emissions test.
- an EGR flow DTC.
- a rough idle.
Show the answer
D. Correct. EGR is normally not flowing at idle, so a system that fails to flow leaves the idle charge as it should be. A rough idle comes from EGR that will not shut off, the opposite fault.
Why not the others
A. Without recirculated exhaust to dilute the charge, combustion runs hotter under load, and spark knock can follow. It could result; a rough idle could not, since the valve is closed at idle anyway.
B. EGR's job is to lower peak combustion temperature, which keeps NOx down; with no flow, NOx under load rises. It could result.
C. The PCM's EGR monitor checks for the expected change, such as in MAP or a flow sensor, when it commands flow; finding none, it can set a flow DTC. It could result.
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A sedan fails the no-load idle emissions test for HC. Fully warm at idle, the five-gas analyzer reads HC 640 ppm, CO 0.2%, CO2 12.1% and O2 3.8%. Which of these could be the cause?
- A failed ignition coil on one cylinder
- A fuel injector that drips when closed
- A coolant sensor reading colder than actual
- A converter that no longer oxidizes HC
Show the answer
A. Correct. A cylinder that does not fire pushes its whole charge, unburned fuel and unused oxygen, into the exhaust, so HC and O2 climb while CO stays low because no combustion took place in that cylinder. A misfire counter or a cylinder balance test will find the dead cylinder.
Why not the others
B. Fuel from a dripping injector is extra fuel the oxygen cannot fully burn, which raises CO and drives O2 down. These readings show low CO and high O2, the opposite of a rich condition; a missing combustion event fits them.
C. A coolant temperature input that reads cold makes the PCM add warm-up enrichment, a rich mixture that raises CO and lowers O2. The low CO and 3.8% O2 here point to a cylinder that is not burning its charge.
D. A converter that has lost its efficiency passes the engine's own HC and O2 through unchanged, but it cannot add oxygen to the exhaust. That much O2 alongside 640 ppm HC means a cylinder is passing its charge unburned.
03Answer key
| Question | Answer | Content area |
|---|---|---|
| Q1 | D | General Powertrain Diagnosis |
| Q2 | B | Computerized Powertrain Controls Diagnosis (including OBD II) |
| Q3 | A | Computerized Powertrain Controls Diagnosis (including OBD II) |
| Q4 | B | Computerized Powertrain Controls Diagnosis (including OBD II) |
| Q5 | D | Ignition System Diagnosis |
| Q6 | A | Fuel Systems and Air Induction Systems Diagnosis |
| Q7 | D | Fuel Systems and Air Induction Systems Diagnosis |
| Q8 | D | Emission Control Systems Diagnosis |
| Q9 | D | Emission Control Systems Diagnosis |
| Q10 | A | I/M Failure Diagnosis |
04Questions by content area
Working on one part of the L1 test? Each content area has its own page with five more free questions.
05More free questions
The L1 study guide covers each content area and how to study for it, and the ASE L1 Advanced Engine Performance Specialist practice test page describes the full L1 question bank. The same free format is available for the other nine tests.
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.