2026 Edition

ASE A9 Practice Test

Take this free ASE A9 Practice Test to study for your ASE A9 certification exam.

The National Institute for Automotive Service Excellence (ASE) identifies Light Vehicle Diesel Engines as an individual service area for certification (A9). The diagnosis and repair of the following subjects are covered:

General Diagnosis
Cylinder Head and Drive Train
Engine Block
Lubrication and Cooling Systems
Air Induction and Exhaust Systems
Fuel Systems

There are 60 questions on the exam, 50 of which are scored, and 75 minutes is given to complete it.

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General Diagnosis

Which diagnostic tool connects to the Data Link Connector and allows the technician to read light duty diesel engine diagnostic trouble codes?

a. DMM (Digital Multimeter)

b. Diagnostic Scan Tool

c. Laptop computer with interface

d. Both b and c

Incorrect. Please choose another answer.

Both b and c are correct because since OBD II (On-Board Diagnostic Level II), the industry has used a 5 character diagnostic trouble code that can only be read with another computer like a scan tool or PC based laptop computer.

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An orange colored liquid is found under the front of the engine, which of the following fluids is leaking?

a. Engine Oil

b. Coolant

c. ATF

d. Washer Fluid

Incorrect. Please choose another answer.

Coolant is orange, pink or green. Engine oil is yellow or brown. ATF is red. Washer fluid is blue.

Cylinder Head and Valve Train Diagnosis and Repair

Two technicians are discussing engine valve seals. Technician A says to apply a light coat of engine oil to new oil seals and push them in. Technician B says you can install the intake oil seal into the exhaust side or install the exhaust oil seal into the intake side because they are interchangeable. Who is right?

a. A only

b. B only

c. Both A and B

d. Neither A nor B

Incorrect. Please choose another answer.

Tech A is right because you need to apply a light coat of engine oil to new oil seals to help ease them in. Tech B is wrong because installing the intake oil seal into the exhaust side or installing the exhaust oil seal into the intake side can cause problems later because the exhaust side is generally hotter than the intake side and the seals are generally different and made of different material and therefore not interchangeable.

Valve adjustment is necessary following the replacement of a:

a. Crankshaft

b. Piston

c. Balancer

d. Cylinder head

Incorrect. Please choose another answer.

Valve adjustment is necessary following the replacement of any valve train or cylinder head.

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Engine Block Diagnosis and Repair

After cleaning a diesel block and crankshaft, what chemical should they be sprayed with?

a. Rust inhibitor

b. Engine oil

c. Anti-seize compound

d. Assembly lube

Incorrect. Please choose another answer.

Spraying the block with a rust inhibiter and wrapping the engine with a plastic bag keeps the engine free from corrosion until you are ready to assemble it.. The other chemicals are not as effective.

When measuring a cylinder for taper wear, which of the following is the MOST likely place for wear?

a. Middle of cylinder

b. Bottom of cylinder

c. At a right angle to top

d. Top of cylinder

Incorrect. Please choose another answer.

The top of the cylinder is where the maximum temperature is found.

Lubrication and Cooling Systems Diagnosis and Repair

Technician A says a weak solution of antifreeze may cause corrosion build-up in the cooling system. Technician B says engine coolants and rust inhibitors last the lifetime of the diesel engine. Who is right?

a. A only

b. B only

c. Both A and B

d. Neither A nor B

Incorrect. Please choose another answer.

Tech A is right because a weak solution of antifreeze may cause corrosion build-up in the cooling system. Tech B is wrong because most original equipment manufacturers (OEM) recommend that the coolant be changed at specified intervals due to a loss in corrosion inhibitor life.

This question has the word EXCEPT. For this question, look for the choice that would NOT fit the described situation. Read the entire question carefully before choosing your answer.

All of the following will happen when an oil filter becomes clogged, EXCEPT:

a. Bypass valve allows dirty oil to lubricate the engine.

b. Oil pump must slow down.

c. Engine operating temperature increases.

d. Oil pump must speed up.

Incorrect. Please choose another answer.

The oil pump will slow down not speed up when an oil filter becomes clogged. A, b and c will occur when an oil filter becomes clogged.

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Air Induction and Exhaust Systems Diagnosis and Repair

A light duty diesel engine has a sever oil leak at the rocker cover. The technician has replaced the rocker cover gasket but the gasket still leaks. Which of the following is the MOST likely cause?

a. Defective cylinder head

b. Defective CDR (crankcase depression regulator) valve

c. Wrong gasket

d. Not enough sealer used

Incorrect. Please choose another answer.

This is the MOST likely cause of the 4 choices. The CDR (crankcase depression regulator) is designed to depress or reduce the crankcase pressure. If it is stuck closed, the pressure in the crankcase increases and pushes the oil out.

A Catalyzed DPF (Diesel Particulate Filter) requires oil with which of the following characteristics?

a. 0% Sulphur

b. High viscosity index

c. Low ash content

d. High alkalinity

Incorrect. Please choose another answer.

Ash is mostly material left over after oil is burned in the combustion chamber. Ash must be physically removed from a DPF since it cannot be regenerated. The other choices do not apply to a DPF.

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Fuel Systems Diagnosis and Repair

A diesel engine with an electronically managed solenoid pump and nozzles has an active diagnostic trouble code for a single cylinder #3 misfire with a rough idle at operating temperature. Which of the following actions could cause the problem?

a. Wrong line on cylinder

b. Open cylinder glow plug

c. Shorted cylinder glow plug

d. High Cetane fuel rating

Incorrect. Please choose another answer.

A short or long replacement line that is different than the original will change the timing in the number 3 cylinder and cause a rough idle or misfire. B is wrong because a glow plug failure does not cause a misfire or rough idle on an engine when hot. C is wrong because a glow plug failure does not cause a misfire or rough idle on an engine when hot. D is wrong because high Cetane fuel does not cause a misfire. High Cetane numbers indicate good ignition quality resulting in a short delay period and low Cetane numbers indicate poor ignition quality that results in a long delay period. Low Cetane numbers can cause a long ignition delay, which can cause a hard starting with white smoke and misfiring cylinders.

The primary fuel system supply side vacuum test should be performed at which area:

a. The outlet pipe for the high-pressure pump looking back to the filter

b. At inlet of the secondary filter looking at the tank

c. At the intake pipe for the high-pressure pump, looking at pump

d. At the inlet side of the primary filter looking at the tank

Incorrect. Please choose another answer.

The intake pipe of the high pressure pump is where the pump will have lowest pressure. A is wrong because positive pressure would be present at the outlet pipe. B is wrong because positive pressure would be present at the inlet of the secondary air filter. D is wrong because positive pressure would be present at the inlet side of the primary filter.