The passing score is determined by a modified Angoff method, which establishes the minimum number of questions a minimally competent technician should answer correctly. This score is then converted to a scaled score for reporting.
1. A technician is diagnosing a vehicle with a rough idle and a strong sulfur-like odor from the exhaust. After checking for diagnostic trouble codes (DTCs) and finding none, the technician observes live data showing that the short-term fuel trim (STFT) for Bank 1 is consistently around +15% to +20%, while the long-term fuel trim (LTFT) is also significantly positive. Which of the following is the most likely cause of these readings and symptoms?
Fuel, Emissions, and Ignition Systems (General)
A.A vacuum leak.
B.A restricted exhaust system.
C.A leaking fuel injector.
D.A faulty oxygen sensor reporting lean.
Show answerAnswer
A. A vacuum leak.
A vacuum leak introduces unmetered air into the engine, causing a lean condition. The engine control unit (ECU) compensates by adding more fuel, resulting in positive short-term and long-term fuel trims. The rich condition from over-compensation can lead to a sulfur smell.
2. A technician is diagnosing a vehicle with a consistent misfire on a single cylinder that occurs at all engine speeds. What is the LEAST likely cause of this specific type of misfire?
Fuel, Emissions, and Ignition Systems (General)
A.A faulty spark plug
B.A clogged fuel injector
C.A faulty ignition coil
D.An intermittent electrical connection to a sensor
Show answerAnswer
D. An intermittent electrical connection to a sensor
A consistent misfire on a single cylinder at all engine speeds points to a hard failure of a component directly responsible for combustion in that cylinder (spark, fuel, or compression). An intermittent electrical connection to a *sensor* (like a crank sensor or cam sensor) would typically affect multiple cylinders or cause an intermittent misfire across the engine, or a no-start condition, rather than a consistent single-cylinder misfire.
3. A vehicle fails an emissions test due to high hydrocarbon (HC) and carbon monoxide (CO) levels. The technician performs a visual inspection and finds no obvious issues. Further diagnosis reveals that the engine is running rich. Which of the following is the most likely cause?
Fuel, Emissions, and Ignition Systems (General)
A.A vacuum leak.
B.A lean air-fuel mixture.
C.A restricted exhaust system.
D.A faulty engine coolant temperature (ECT) sensor reporting a cold engine.
Show answerAnswer
D. A faulty engine coolant temperature (ECT) sensor reporting a cold engine.
High HC and CO emissions, especially when the engine runs rich, indicate incomplete combustion. If the ECT sensor falsely reports a cold engine, the ECU will enrich the fuel mixture for warm-up, leading to a consistently rich condition and high HC/CO.
4. A client complains that their vehicle occasionally stalls when coming to a stop, especially after driving for a while. There are no DTCs stored. The technician suspects an issue with the idle air control (IAC) valve. What is the most appropriate diagnostic step to take next?
Fuel, Emissions, and Ignition Systems (General)
A.Check for vacuum leaks around the intake manifold.
B.Perform a compression test on all cylinders.
C.Replace the IAC valve immediately.
D.Monitor IAC valve position and engine RPM using a scan tool.
Show answerAnswer
D. Monitor IAC valve position and engine RPM using a scan tool.
Before replacing any part, it's crucial to confirm its malfunction. Monitoring the IAC valve's commanded position and actual engine RPM with a scan tool will show if the valve is responding correctly to the ECM's commands and if it's effectively controlling idle speed. This helps confirm if the IAC is the problem or if another issue (like a vacuum leak) is affecting idle.
5. A vehicle experiences a misfire on cylinder #3. The technician swaps the ignition coil from cylinder #3 with cylinder #1, and the misfire moves to cylinder #1. What is the most likely cause of the original misfire?
Fuel, Emissions, and Ignition Systems (General)
A.Low compression in cylinder #3
B.A faulty ignition coil that was originally on cylinder #3
C.A faulty spark plug in cylinder #3
D.A clogged fuel injector in cylinder #3
Show answerAnswer
B. A faulty ignition coil that was originally on cylinder #3
By swapping the ignition coil and observing the misfire move with the coil, the technician has successfully isolated the faulty component. This indicates the ignition coil itself was the cause of the misfire on cylinder #3.
6. A vehicle fails an emissions test with high NOx (nitrogen oxides) levels. The technician confirms the engine is not running lean and there are no misfires. Which of the following components should be investigated as the most likely cause?
Fuel, Emissions, and Ignition Systems (General)
A.The catalytic converter
B.The oxygen sensor
C.The exhaust gas recirculation (EGR) valve
D.The fuel pressure regulator
Show answerAnswer
C. The exhaust gas recirculation (EGR) valve
High NOx emissions are primarily caused by high combustion temperatures. The Exhaust Gas Recirculation (EGR) system is designed to lower combustion temperatures by introducing inert exhaust gases into the intake manifold. If the EGR valve is stuck closed or otherwise not functioning, combustion temperatures will be too high, leading to excessive NOx.
7. A client complains that their vehicle fails to start. The technician connects a spark tester to a spark plug wire and cranks the engine, observing no spark. Next, the technician uses a multimeter to check for voltage at the positive (+) terminal of the ignition coil. The multimeter reads battery voltage. What should the technician check next to diagnose the no-start condition?
Fuel, Emissions, and Ignition Systems (General)
A.The resistance of the spark plug wires.
B.The primary winding resistance of the ignition coil.
C.The condition of the spark plugs.
D.The triggering signal from the ignition module or engine control unit (ECU).
Show answerAnswer
D. The triggering signal from the ignition module or engine control unit (ECU).
If battery voltage is present at the coil's positive terminal, the power supply to the coil is good. The next step is to determine if the coil is being commanded to fire. This involves checking the ground-side switching (triggering signal) from the ignition module or ECU.
8. A vehicle fails an emissions test due to high HC (hydrocarbons) and CO (carbon monoxide) levels. The technician checks for DTCs and finds none. Which of the following is the most likely cause?
Fuel, Emissions, and Ignition Systems (General)
A.A clogged exhaust gas recirculation (EGR) valve
B.An excessively rich air-fuel mixture or engine misfires
C.A lean air-fuel mixture due to a vacuum leak
D.A faulty catalytic converter
Show answerAnswer
B. An excessively rich air-fuel mixture or engine misfires
High HC (unburned fuel) and high CO (partially burned fuel) indicate an incomplete combustion process. This is typically caused by an excessively rich air-fuel mixture (too much fuel, not enough air) or by engine misfires where fuel is not burned at all. The absence of DTCs suggests the problem is not severe enough to trigger a fault code, or is a borderline condition.
9. A vehicle exhibits a prolonged crank time but eventually starts. The technician connects a fuel pressure gauge and observes that the fuel pressure drops rapidly after the engine is shut off. What is the most likely cause of this condition?
Fuel, Emissions, and Ignition Systems (General)
A.A leaking fuel injector or faulty fuel pump check valve
B.A faulty fuel pressure regulator that is stuck open
C.A restricted fuel filter
D.An over-pressurized fuel system
Show answerAnswer
A. A leaking fuel injector or faulty fuel pump check valve
A rapid drop in fuel pressure after the engine is shut off indicates a leak in the fuel system, allowing pressure to bleed off. The most common culprits for this symptom are a leaking fuel injector or a faulty check valve within the fuel pump assembly.
10. A vehicle owner reports that their check engine light (CEL) illuminates intermittently, and the engine sometimes hesitates under hard acceleration. A scan tool reveals a pending Diagnostic Trouble Code (DTC) P0171 (System Too Lean, Bank 1). The technician inspects the engine and finds no obvious vacuum leaks. What component should the technician investigate next?
Fuel, Emissions, and Ignition Systems (General)
A.The exhaust gas recirculation (EGR) valve.
B.The mass air flow (MAF) sensor.
C.The evaporative emission (EVAP) purge valve.
D.The catalytic converter.
Show answerAnswer
B. The mass air flow (MAF) sensor.
DTC P0171 indicates a lean condition. While vacuum leaks are a common cause, a faulty MAF sensor that under-reports airflow can also cause the ECU to inject less fuel than needed, leading to a lean condition and hesitation under acceleration due to insufficient fuel.
11. A vehicle with a coil-on-plug (COP) ignition system has a misfire on cylinder #4. The technician performs an amperage ramp test on the ignition coil for cylinder #4 using an oscilloscope and observes a sudden, sharp drop to zero amps during the dwell period, followed by an immediate rise. What does this waveform indicate?
Fuel, Emissions, and Ignition Systems (General)
A.A shorted primary winding within the ignition coil.
B.An intermittent open circuit in the ground control (driver) for the coil.
C.An open primary circuit to the ignition coil.
D.Excessive resistance in the coil's power supply circuit.
Show answerAnswer
B. An intermittent open circuit in the ground control (driver) for the coil.
An amperage ramp test shows the current building in the primary winding during the dwell period. A sudden, sharp drop to zero amps followed by an immediate rise indicates an intermittent interruption in the primary circuit's ground path. The current builds, is suddenly interrupted, and then resumes if the connection is restored. This points to an intermittent open in the coil driver or its wiring.
12. A technician observes that a vehicle's oxygen sensor (O2S) waveform for Bank 1 Sensor 1 (B1S1) is 'stuck rich' (consistently high voltage). Which of the following is the most likely cause?
Fuel, Emissions, and Ignition Systems (General)
A.A leaking fuel injector on Bank 1.
B.A faulty catalytic converter on Bank 1.
C.A vacuum leak at the intake manifold.
D.A restricted exhaust system.
Show answerAnswer
A. A leaking fuel injector on Bank 1.
An O2S B1S1 (upstream) waveform that is 'stuck rich' (high voltage, typically above 0.8V for a zirconia sensor) indicates that the engine is genuinely running with an excessively rich air-fuel mixture. A leaking fuel injector would continuously deliver too much fuel to one or more cylinders on that bank, causing the exhaust to have low oxygen content, which the O2 sensor interprets as rich.
13. A technician is performing a basic ignition system diagnosis on a vehicle with a no-start condition. The technician confirms there is fuel pressure and proper engine cranking. What is the next logical step to diagnose the ignition system?
Fuel, Emissions, and Ignition Systems (General)
A.Inspect the fuel injectors.
B.Perform a compression test.
C.Check for spark at the spark plugs.
D.Replace the ignition coil.
Show answerAnswer
C. Check for spark at the spark plugs.
In a no-start condition where fuel and cranking are confirmed, the next logical step in ignition system diagnosis is to verify the presence of spark. This directly checks if the ignition system is producing the necessary high voltage to ignite the air-fuel mixture.
14. A vehicle with distributorless ignition system (DIS) experiences a misfire on cylinder #2. The technician determines the spark plug is good. Using a scope, the technician observes that the primary ignition coil waveform for cylinder #2 is flat. What is the most likely cause?
Fuel, Emissions, and Ignition Systems (General)
A.A grounded spark plug wire for cylinder #2.
B.A faulty engine control module (ECM) providing an incorrect trigger signal.
C.An open in the primary circuit to the ignition coil for cylinder #2.
D.A shorted secondary (high voltage) winding in the coil pack.
Show answerAnswer
C. An open in the primary circuit to the ignition coil for cylinder #2.
A flat primary ignition coil waveform indicates that the coil is not being energized and de-energized, meaning no current is flowing through the primary winding. This is most likely caused by an open circuit in the primary side, preventing the ECM from switching the coil's ground or power. A shorted secondary would still show primary activity, and a faulty ECM would likely affect multiple cylinders or show a different waveform.
15. A technician is diagnosing a vehicle that intermittently stalls at idle and experiences poor fuel economy. A scan tool shows that the engine control unit (ECU) is frequently adjusting the commanded idle speed. The technician suspects an issue with the idle air control (IAC) system. Which of the following is the most appropriate next diagnostic step?
Fuel, Emissions, and Ignition Systems (General)
A.Perform a compression test on all cylinders.
B.Test the resistance of the oxygen sensor.
C.Inspect the throttle body for carbon buildup around the throttle plate.
D.Check the fuel pressure at the fuel rail.
Show answerAnswer
C. Inspect the throttle body for carbon buildup around the throttle plate.
If the IAC system is struggling to maintain idle speed, it often indicates insufficient airflow at idle. Carbon buildup around the throttle plate can restrict the bypass air controlled by the IAC, forcing the ECU to constantly adjust and potentially leading to stalls and poor idle control.
16. A vehicle exhibits an intermittent misfire and hesitation under acceleration. The technician uses a scan tool and notices that the short-term fuel trim (STFT) is consistently negative (-15% or more) for Bank 1. What does this indicate?
Fuel, Emissions, and Ignition Systems (General)
A.The engine is running rich and the ECM is removing fuel.
B.There is an exhaust leak before the upstream oxygen sensor.
C.The engine is running lean and the ECM is adding fuel.
D.The oxygen sensor for Bank 1 is faulty and biased lean.
Show answerAnswer
A. The engine is running rich and the ECM is removing fuel.
A consistently negative short-term fuel trim (STFT) indicates that the engine control module (ECM) is detecting an excessively rich air-fuel mixture and is actively trying to remove fuel to compensate. If the ECM is removing a significant amount of fuel (-15% or more), it suggests a problem causing the engine to run rich, leading to potential misfires and hesitation.
17. A technician is diagnosing a vehicle with a strong fuel smell and black smoke from the exhaust. A scan tool shows that the engine's oxygen sensor (O2S) for Bank 1 Sensor 1 (upstream) is consistently reading a high voltage (0.8-0.9V). What does this O2 sensor reading indicate about the air-fuel mixture?
Fuel, Emissions, and Ignition Systems (General)
A.A rich air-fuel mixture.
B.An optimal stoichiometric air-fuel mixture.
C.A fluctuating air-fuel mixture.
D.A lean air-fuel mixture.
Show answerAnswer
A. A rich air-fuel mixture.
A consistently high voltage (0.8-0.9V) from a zirconium dioxide (ZrO2) oxygen sensor indicates a rich air-fuel mixture, meaning there is insufficient oxygen in the exhaust. This aligns with the symptoms of a strong fuel smell and black smoke.
18. A vehicle owner complains of a strong fuel odor, especially after refueling. The technician performs a visual inspection and finds no external fuel leaks. Which system component should the technician investigate next?
Fuel, Emissions, and Ignition Systems (General)
A.The fuel pressure regulator
B.The evaporative emission (EVAP) control system
C.The exhaust gas recirculation (EGR) valve
D.The oxygen sensor
Show answerAnswer
B. The evaporative emission (EVAP) control system
A strong fuel odor, especially after refueling, without external leaks, is a classic symptom of a fault in the evaporative emission (EVAP) control system. This system is designed to capture and store fuel vapors from the tank and release them into the engine to be burned.
19. A technician is performing a cooling system pressure test. The system is pressurized to 15 PSI. After 10 minutes, the pressure has dropped to 10 PSI, and no external leaks are visible. The engine oil appears normal, but the technician notices a strong sweet smell from the exhaust and periodic white smoke, especially on startup. What is the most likely cause of the pressure drop?
Lubrication and Cooling Systems
A.Internal head gasket leak into the combustion chamber
B.External heater core leak
C.Cracked engine block
D.Faulty radiator cap
Show answerAnswer
A. Internal head gasket leak into the combustion chamber
A pressure drop during a cooling system pressure test with no external leaks, combined with a sweet smell and white smoke from the exhaust (especially on startup), strongly indicates an internal head gasket leak where coolant is entering the combustion chamber and being burned.
20. A technician is performing a multi-angle valve seat cut on a cylinder head. The service manual specifies a 30-degree top cut, a 45-degree seat cut, and a 60-degree throat cut. What is the primary purpose of the 60-degree throat cut?
Cylinder Head and Valve Train
A.To improve valve sealing.
B.To improve airflow into and out of the cylinder.
C.To reduce valve guide wear.
D.To adjust the effective valve spring installed height.
Show answerAnswer
B. To improve airflow into and out of the cylinder.
The 60-degree throat cut, also known as the bottom cut or bowl cut, is designed to open up the area below the valve seat, improving the transition for airflow into (for intake) or out of (for exhaust) the cylinder. This enhances volumetric efficiency.
21. A technician is performing a visual inspection of a camshaft with a dial indicator. The maximum lift measured on an intake lobe is 0.250 inches, and the base circle runout is within specifications. The service manual specifies an intake valve lift of 0.400 inches. What is the most likely cause for the discrepancy in measured lift?
Cylinder Head and Valve Train
A.The dial indicator is faulty.
B.The camshaft lobe is excessively worn.
C.The valve spring is weak.
D.The rocker arm ratio is incorrect.
Show answerAnswer
B. The camshaft lobe is excessively worn.
A significant reduction in camshaft lobe lift (0.250 inches measured vs. 0.400 inches specified) with a normal base circle runout strongly indicates that the camshaft lobe itself is excessively worn. This directly reduces the amount the valve is lifted.
22. A customer complains of a loss of engine power, rough idle, and increased oil consumption. During a cylinder leak-down test, the technician hears air escaping from the exhaust system when testing cylinder #3. Which of the following components is most likely faulty?
Cylinder Head and Valve Train
A.An intake valve that is not sealing correctly.
B.An exhaust valve that is not sealing correctly.
C.A blown head gasket between cylinders #3 and #4.
D.A worn piston ring.
Show answerAnswer
B. An exhaust valve that is not sealing correctly.
Air escaping from the exhaust system during a cylinder leak-down test indicates that the exhaust valve in that cylinder is not properly sealing, allowing combustion gases to escape into the exhaust manifold.
23. A technician is diagnosing an overhead valve (OHV) engine with a persistent ticking noise that increases with engine RPM. The noise is isolated to a specific cylinder. After removing the valve cover, the technician observes that the pushrod on the affected cylinder has excessive vertical play when the valve is closed. Which component is the most likely cause?
Cylinder Head and Valve Train
A.Collapsed hydraulic lifter
B.Bent pushrod
C.Worn rocker arm pivot
D.Worn camshaft lobe
Show answerAnswer
A. Collapsed hydraulic lifter
Excessive vertical play in the pushrod on an OHV engine with a ticking noise indicates that the hydraulic lifter is not maintaining its proper length, allowing for excessive valve lash. A collapsed hydraulic lifter cannot pump up and take up this clearance.
24. A vehicle owner reports that their engine frequently runs hot, but the temperature gauge sometimes drops to cold while driving, then returns to a high reading. The coolant level is full. What is the most likely cause of this intermittent temperature gauge behavior?
Lubrication and Cooling Systems
A.A stuck-open thermostat
B.A faulty engine coolant temperature (ECT) sensor
C.An airlock in the cooling system
D.A failing water pump impeller
Show answerAnswer
C. An airlock in the cooling system
An airlock in the cooling system can cause the ECT sensor to be exposed to air instead of coolant. Air heats up and cools down much faster than liquid coolant, causing rapid fluctuations in the sensor reading, leading to the gauge jumping from hot to cold and back.
25. A technician is performing a charging system test. With the engine running at 2000 RPM and all accessories off, the battery voltage is 14.1 volts. With the headlights, blower motor on high, and rear defroster activated, the voltage drops to 12.8 volts. The battery is fully charged. What does this indicate?
Engine Electrical Systems (General)
A.The alternator is overcharging the battery.
B.There is a parasitic draw on the battery.
C.The voltage regulator is faulty, causing excessive output.
D.The alternator's maximum output current is insufficient for the electrical load.
Show answerAnswer
D. The alternator's maximum output current is insufficient for the electrical load.
A healthy charging system should maintain voltage above 13.0 volts (ideally 13.5-14.5 volts) even under heavy electrical load. A drop to 12.8 volts indicates that the alternator cannot meet the demand, meaning its current output (amperage) is insufficient to maintain system voltage under load. The battery is then supplying the deficit.
Positive fuel trim indicates the engine control unit (ECU) is adding fuel to compensate for a lean condition, often caused by unmetered air or insufficient fuel delivery.
STFT and LTFT measure fuel adjustments.
Positive trim means ECU is adding fuel.
Common causes include vacuum leaks, low fuel pressure, or dirty MAF sensor.
High levels of unburned hydrocarbons (HC) and carbon monoxide (CO) in exhaust indicate incomplete combustion, often due to an overly rich air-fuel mixture.
HC = unburned fuel, CO = partially burned fuel.
High levels often mean engine is running rich.
Common causes: faulty sensors, leaking injectors, incorrect fuel pressure.
The process of evaluating the function of the IAC valve, which regulates airflow past the throttle plate to control engine idle speed, typically by monitoring its operation with a scan tool.
Commonly causes rough idle or stalling.
ECM controls its position based on engine load and temperature.
A diagnostic technique where a suspected faulty component (e.g., spark plug, ignition coil, injector) from a misfiring cylinder is swapped with a known good component from another cylinder to see if the misfire follows the component.
Diagnosing a no-spark condition requires a systematic approach, starting with confirming power to the ignition coil and then checking for the triggering signal.
No spark means no combustion.
Check power to coil first.
Then check for triggering signal (ground-side switch).
DTC P0171 indicates that the engine control unit (ECU) is detecting a lean air-fuel mixture in Bank 1, meaning there's too much air or not enough fuel.
Indicates System Too Lean, Bank 1.
Common causes: vacuum leaks, low fuel pressure, faulty MAF sensor.
Leads to ECU adding fuel (positive fuel trims) to compensate.
An oscilloscope waveform displaying the current flow (amperage) through the primary winding of an ignition coil during the dwell period, used to diagnose coil and circuit integrity.
Shows current build-up (ramp) and dwell time.
Abnormalities indicate issues with coil, power, ground, or driver.
The systematic process of identifying the fault within the ignition system when an engine cranks but does not start, typically starting with verifying spark presence.
Requires three elements: spark, fuel, compression.
After confirming fuel and cranking, spark is the next check.
Can be checked with a spark tester or by observing a plug.
Diagnosis of IAC system issues often involves checking for physical obstructions, vacuum leaks, and proper sensor/actuator operation that affect idle air flow.
IAC controls idle speed by regulating bypass air.
Stalling/poor idle are common symptoms.
Carbon buildup is a frequent cause of IAC malfunction.
A scan tool reading indicating the engine control module (ECM) is actively reducing the amount of fuel delivered to compensate for a detected rich air-fuel mixture.
Measured as a percentage, typically +/- 20%.
Negative values mean fuel removal, positive values mean fuel addition.
Consistently high negative values (e.g., -15% or more) indicate a significant rich condition.
A fault within the Evaporative Emission Control System that leads to the release of fuel vapors into the atmosphere, often causing a noticeable fuel odor.
Designed to prevent fuel vapor escape.
Common issues include loose gas caps, faulty valves, or cracked hoses.
Often triggers a 'Check Engine' light with EVAP-related DTCs.
An airlock is a pocket of air trapped in the cooling system that can impede coolant flow and cause erratic temperature readings, especially when the ECT sensor is exposed to air instead of liquid coolant.
Intermittent temperature gauge fluctuations (hot to cold)
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