2025 Edition

ASE L2 Practice Test

ASE identifies Electronic Diesel Engine Diagnosis Specialist as an individual service area (L2). The electronic diesel engine diagnosis area involves the following components:

• General Inspection and Diagnosis

• Electronic Engine Controls Diagnosis

• Air Exhaust Systems Diagnosis
• Fuel Systems Diagnosis
• Emission Systems Diagnosis

This free sample ASE L2 Practice Test below consists of 8 questions organized within 4 sections.

Tests.com's ASE L2 Practice Test was written by Dr. John Kershaw, an ASE test prep expert: ASE test question developer; ASE Certified Master Technician; ASE Master Truck Technician; and Auto Industry Technical Instructor. Dr. Kershaw was among the first to receive ASE certification in 1972.

For the complete practice test, flash cards, exam review and testing tips, check out Tests.com's Complete 100 Question L2 Practice Test, a 100 question practice test kit for the ASE L2 Exam.

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General Inspection and Diagnosis

1.	A diesel truck equipped with a JWAC (Jacket Water Aftercooler) has low power under load. Which of the following could be the cause?
		a. Leak in turbocharger inlet piping
		b. Using an off road grade of fuel
		c. Leaking JWAC (Jacket Water Aftercooler)
		d. Open in EGR sensor circuit
A leak in the JWAC will raise the intake air temperature, which lowers engine power because the air is less dense and produces less energy. A is wrong because a leak in the turbo inlet manifold affect oil leakage but not power output. B is wrong because the off-road Diesel #2 and fuel oil # 2 differ only in the tax applied at the time of sale. Both are dyed red. D is wrong because EGR has no effect on diesel engine at higher throttle openings, because it is when closed at WOT and open at idle. Incorrect answer. Please choose another answer.

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2.	A medium duty truck using a HEUI (hydraulically actuated electronically controlled unit injector) fuel system has a misfire code when hot. Technician A says worn HEUI injectors could be the cause. Technician B says a shorted glow plug could be the cause. Who is right?
		a. A only
		b. B only
		c. Both A and B
		d. Neither A nor B
Tech A is right because when an injector is worn, it will leak fuel internally and reduce power or cause a misfire. Tech B is wrong because a shorted glow plug will only cause a problem when the engine is cold. Incorrect answer. Please choose another answer.

Electronic Engine Controls Diagnosis

3.	The technician gets an infinite ohms reading on the ohmmeter function of the DMM (Digital Multimeter), when measuring resistance in a circuit. Which of the following could be the cause?
		a. Circuit is open
		b. Corroded terminal
		c. Circuit is in operation
		d. Circuit is complete
An infinity reading indicates an open circuit because the reading is infinite. B is wrong because a corroded terminal would cause high resistance not infinity. C is wrong because with infinite resistance the circuit cannot operate. D is wrong because infinite resistance indicates an open, so the circuit is incomplete. Incorrect answer. Please choose another answer.

4.	Technician A says you do not have to solder a repaired wire. Technician B says wire insulation damage is sometimes found where the wire passes through an opening in a metal part. Who is right?
		a. A only
		b. B onlly
		c. Both A and B
		d. Neither A nor B
Tech B is right. Wire insulation damage can take place when the wire passes through an opening in a metal part. Tech A is wrong because the standard repair practice is to solder wiring repairs. Incorrect answer. Please choose another answer.

Air Exhaust Systems Diagnosis

5.	Which of the following measuring tools is MOST likely used to measure the temperature of the diesel exhaust gases?
		a. MDI (Multiple Diagnostic Interface)
		b. Exhaust manometer
		c. Diagnostic scan tool
		d. Exhaust pyrometer
Incorrect answer. Please choose another answer.

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6.	An active diagnostic trouble code is found on a heavy duty diesel engine with VVT (Variable Valve Timing). Technician A says the camshaft actuator is opening the exhaust valve too early. Technician B says a slow response could cause the engine not to meet exhaust emissions. Who is right?
		a. A only
		b. B only
		c. Both A and B
		d. Neither A nor B
Incorrect answer. Please choose another answer.

Fuel Systems Diagnosis

7.	The engine cranks on a diesel, but will not start. During diagnosis, the ECM (Electronic Control Module) will not communicate with the diagnostic tool. What is the MOST LIKELY cause?
		a. Failed data link connector
		b. Failed engine speed/timing sensor
		c. Low ECM supply voltage
		d. Incorrect diagnostic tool software
Incorrect answer. Please choose another answer.

8.	Technician A says the diesel plastic fuel lines are separated by removing a clip and disconnecting (squeezing) the secondary latch on the line fitting. Technician B says an air leak at the line connecting the tank pick-up to the transfer pump can cause low-power and hard starting. Who is right?
		a. A only
		b. B only
		c. Both A and B
		d. Neither A nor B
Incorrect answer. Please choose another answer.

Emission Systems Diagnosis

9.	Which of the following tools best measures backpressure restrictions?
		a. Mercury manometer
		b. Magnahelic gauge
		c. 0-160 psi pressure gauge
		d. Vacuum gauge
A magnahelic gauge or water manometer compares very small amounts of pressure differential. The limits to acceptable backpressure in most diesel engines is 3" of mercury or 14" of water column as measured with a magnahelic gauge or water manometer. Conspicuous performance complaints are noticeable at approximately 30 - 40" of water. For this reason, the exhaust system is designed and must be maintained to minimize exhaust backpressure. Answer A is wrong because the mercury manometer is not sensible enough. Answer C is wrong because the pressure is very small. Answer D is wrong because a vacuum gauge is like mercury manometer and not sensible enough. Incorrect answer. Please choose another answer.

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10.	The EGR (Exhaust Gas Recirculation) system is disabled under what engine conditions?
		a. Heavy acceleration
		b. DPF (Diesel Particulate Filter) regeneration
		c. High Barometric Pressure (BARO)
		d. High engine temperature
During idle, cold weather or engine operation, minimal or no EGR flow usually takes place. During warm-up, condensation of corrosive exhaust can take place in EGR coolers so flow is minimized or disabled during that period. When high coolant temperatures exist, the EGR flow may also be disabled since cooling of exhaust gases in EGR coolers may contribute to overheated engine operation. High altitude or low barometric pressures may also be a condition to disable EGR operation since EGR flow under these conditions could significantly deteriorate combustion quality and engine performance. Incorrect answer. Please choose another answer.

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Dr. John Kershaw
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ASE Master Truck Technician

Auto Industry Technical Instructor

Among first ASE Certified in 1972

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