A bus burned 100 gallons of diesel fuel last week. About how much diesel exhaust fluid (DEF) would its SCR system typically have used?Wikipedia, Diesel exhaust fluid, Background. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
A bus on a short stop-and-go route keeps loading its particulate filter with soot. At what speeds does regeneration occur?Wikipedia, Diesel particulate filter, Mode of action. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
A cold diesel engine gives off blue and white smoke and lacks power until it has warmed. What is happening in the cylinders?Wikipedia, Diesel exhaust, Variation with engine conditions. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
A fleet sends an oil sample from each bus at every service. Why are the results tracked over the life of the engine?Wikipedia, Oil analysis, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
Show all 55 questions (51 more)
General engine diagnosis, continued
A hand-held tachometer on the fuel pump shaft of a four-cycle engine reads 900 rpm. What is the engine speed?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Engine Testing, tachometer. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
A power complaint is checked on a chassis dynamometer and not on the road. How is the bus loaded there?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Horsepower and Horsepower Ratings, Brake Horsepower. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
A regular clicking from the engine grows louder as engine speed rises. What is the usual cause, by the Navy manual?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Engine Noises, Valve and Tappet Noise. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
A school bus moves from a sea-level district to one at 4,000 feet and the driver reports less power. By the Navy manual's rule, how much engine output is lost to altitude alone?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Power Losses and Failure. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
A technician needs the engine family to look up service information. Which heading does the federal rule require on the engine's permanent label?40 CFR 1036.135, paragraphs (b) and (c). Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
An engine is being rebuilt. Under 40 CFR 1068.120, what must be true of the parts that are installed?40 CFR 1068.120, paragraph (d)(1). Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
An oil analysis report lists the base number (BN) of the engine oil. What does BN measure?Wikipedia, Total base number, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
On an OBD-II vehicle, a scan tool clears the emission-related diagnostic information. Besides the stored trouble codes, what is erased?Wikipedia, On-board diagnostics, Mode of operation/OBD services. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
Slime is found in a fuel tank that has water at the bottom. What does water in diesel fuel encourage?Wikipedia, Microbial contamination of diesel fuel, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
Soot has built up in the diesel particulate filter. What does that buildup cause that can lead to engine problems?Wikipedia, Diesel particulate filter, History. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
Why is diesel exhaust fluid (DEF) stored and transported in special containers?Wikipedia, Diesel exhaust fluid, Safety and storage. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), A. General Engine Diagnosis (15 of 55)
In-chassis engine inspection and repair 6
A valve lifter is out of the engine. A straightedge is laid across its bottom and light shows between the two. What does the Navy manual say?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Valve Lifter Servicing. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), B. In-Chassis Engine Inspection and Repair (6 of 55)
At what engine temperature are most overhead valves adjusted, by the Navy manual?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Valve Adjustments, Overhead Valves. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), B. In-Chassis Engine Inspection and Repair (6 of 55)
How is a camshaft checked for alignment, by the Navy manual?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Camshaft Checking. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), B. In-Chassis Engine Inspection and Repair (6 of 55)
New camshaft bearings are being installed. What must be aligned, by the Navy manual?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Camshaft Bearing Replacement. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), B. In-Chassis Engine Inspection and Repair (6 of 55)
The valves are out of a cylinder head for service. Where does each valve go at reassembly, by the Navy manual?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Valve Removal. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), B. In-Chassis Engine Inspection and Repair (6 of 55)
Valves are to be set on one cylinder with the engine stopped. On the cylinder paired with it, the exhaust valve is just closing and the intake valve just opening. Where is the cylinder to be set?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Valve Adjustments, Valves in Block. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), B. In-Chassis Engine Inspection and Repair (6 of 55)
Lubrication and cooling systems 7
A coolant temperature sensor is an NTC (negative temperature coefficient) thermistor. How does its resistance change as the engine warms up?Wikipedia, Thermistor, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), C. Lubrication and Cooling Systems Diagnosis and Repair (7 of 55)
An air-cooled oil cooler takes the form of a small radiator. What can it not do, by TM 9-8000?US Army TM 9-8000, Principles of Automotive Vehicles (1985), chapter 8, paragraph 8-14, Oil Coolers. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), C. Lubrication and Cooling Systems Diagnosis and Repair (7 of 55)
An engine has no oil cooler. How is the heat that the oil picks up removed, by TM 9-8000?US Army TM 9-8000, Principles of Automotive Vehicles (1985), chapter 8, paragraph 8-3, Oil as a Coolant. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), C. Lubrication and Cooling Systems Diagnosis and Repair (7 of 55)
Coolant is recovered when a cooling system is drained and flushed. How is the ethylene glycol in it classed?Wikipedia, Antifreeze, Ethylene glycol, Other industrial uses. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), C. Lubrication and Cooling Systems Diagnosis and Repair (7 of 55)
How is an engine oil pump typically driven?Wikipedia, Oil pump (internal combustion engine), Pump drive. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), C. Lubrication and Cooling Systems Diagnosis and Repair (7 of 55)
One serpentine belt drives the alternator, the water pump and the power steering pump. What happens if it breaks?Wikipedia, Serpentine belt, Disadvantages. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), C. Lubrication and Cooling Systems Diagnosis and Repair (7 of 55)
Why are corrosion inhibitors added to automotive antifreeze?Wikipedia, Antifreeze, Automotive and internal combustion engine use. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), C. Lubrication and Cooling Systems Diagnosis and Repair (7 of 55)
Air induction and exhaust systems 9
A charge air cooler is plugged with dirt and the intake air runs hot. By the Navy manual, how much power may engineers deduct for hot intake air?US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, Engine Troubleshooting and Overhaul, Power Losses and Failure. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
A DEF tank heater is being tested. Why should the heater not exceed 50 to 60 °C?Wikipedia, Diesel exhaust fluid, Operation in winter time. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
A failed turbocharger is replaced in a way that significantly increases the service life of the engine. How does 40 CFR 1068.120 class that work?40 CFR 1068.120, paragraph (b). Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
A turbocharger seal has failed and engine oil has reached the particulate filter. What may the filter then need?Wikipedia, Diesel particulate filter, Mode of action. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
Cooled EGR parts go through repeated, rapid changes in temperature. What can that cause?Wikipedia, Exhaust gas recirculation, Diesel engines. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
In active regeneration, what does the extra fuel injector in the exhaust stream do?Wikipedia, Diesel particulate filter, Mode of action. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
What is a glow plug?Wikipedia, Glow plug, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
Which metals does a diesel oxidation catalyst (DOC) contain as its catalyst?Wikipedia, Catalytic converter, Diesel engines. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
Why is the recirculated exhaust gas usually cooled in a modern diesel engine?Wikipedia, Exhaust gas recirculation, Diesel engines. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), D. Air Induction and Exhaust Systems Diagnosis and Repair (9 of 55)
Fuel system 15
A diesel engine runs away on its own oil and the key will not stop it. How can a runaway diesel be stopped?Wikipedia, Diesel engine runaway, Stopping a runaway engine. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
A diesel fill pipe is replaced on a tank of more than 25 gallons. What fill rate must the fill pipe and vents allow without spillage under 49 CFR 393.67?49 CFR 393.67, paragraph (c)(7)(ii). Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
A drain fitting is installed in the bottom of a fuel tank. How far below the lowest part of the tank may it extend under 49 CFR 393.67?49 CFR 393.67, paragraph (c)(4), Drains and bottom fittings. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
A fleet switches its buses to a biodiesel blend. What is recommended shortly after the switch?Wikipedia, Biodiesel, Applications. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
A fuel tank is relocated on a chassis. Under 49 CFR 393.65, what must fuel spilled while the tank is being filled not touch?49 CFR 393.65, paragraph (b)(3). Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
A larger fuel tank is fitted to a bus chassis. How far out may any part of the fuel system reach under 49 CFR 393.65?49 CFR 393.65, paragraph (b)(1). Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
HEUI injectors are no longer operated by the camshaft. What did that let them do?Wikipedia, Unit injector, Hydraulically actuated electronic (HEUI) development and applications. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
In a common rail engine, the control unit injects a small amount of fuel just before the main injection. What is this pilot injection for?Wikipedia, Common rail, Principles. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
Under FMVSS No. 301, the fuel system of a school bus over 10,000 pounds GVWR is tested with a moving contoured barrier. Up to what barrier speed?49 CFR 571.301, S6.5, Moving contoured barrier crash. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
What did the refining process that cut diesel sulfur to 15 ppm also do to the fuel?Wikipedia, Ultra-low-sulfur diesel, Lubricity. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
What did third-generation common rail systems bring in for increased precision?Wikipedia, Common rail, Principles. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
What does 49 CFR 393.65 say about feeding fuel to the injectors by gravity or syphon?49 CFR 393.65, paragraph (d), Gravity or syphon feed prohibited. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
What is a typical pressure in the fuel rail of a common rail system?Wikipedia, Common rail, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
What usually drives the plunger pump of a unit injector?Wikipedia, Unit injector, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
Which two types of mass airflow sensor are in common use on automotive engines?Wikipedia, Mass flow sensor, lead section. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), E. Fuel System Diagnosis and Repair (15 of 55)
Starting system 3
On which type of lead-acid battery can the electrolyte level be checked and pure water added?Wikipedia, Lead–acid battery, Charging. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), F. Starting System Diagnosis and Repair (3 of 55)
Two batteries sit in a bus through a hard freeze. One is fully charged and one is nearly flat. Which battery's electrolyte is the likelier to freeze?Wikipedia, Lead–acid battery, Effect of charge level on freezing point. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), F. Starting System Diagnosis and Repair (3 of 55)
Why must a starter motor not be cranked without pause for minutes at a time?Wikipedia, Starter (engine), History. Outline: ASE School Bus Study Guide (2026), Diesel Engines (Test S2), F. Starting System Diagnosis and Repair (3 of 55)
How this list was compiled: Our own questions, written to the six content areas of the Diesel Engines (Test S2) test specifications and task list in the ASE School Bus Study Guide (copyright 2026, https://ase.com/wp-content/uploads/ASE_SchoolBus_Study_Guide.pdf, read 2026-10-13): general engine diagnosis 15 of 55 scored questions, in-chassis engine inspection and repair 6, lubrication and cooling systems 7, air induction and exhaust systems 9, fuel system 15, starting system 3. The bank holds the same 15, 6, 7, 9, 15 and 3. Every fact was read on 2026-10-13 in the public text the question names: US Navy NAVEDTRA 14050, Construction Mechanic, Advanced (1992), chapter 3, and US Army TM 9-8000, Principles of Automotive Vehicles (1985), chapter 8, both in the copies at archive.org; 49 CFR 393.65, 393.67 and 571.301 and 40 CFR 1036.135 and 1068.120 at ecfr.gov; and Wikipedia articles on diesel exhaust fluid, particulate filters, exhaust gas recirculation, common rail, unit injectors, fuels, oils, coolant, sensors, belts, batteries and starters. The sample questions printed in the study guide were not used. Licence: Questions, choices and explanations are original work of this site. NAVEDTRA 14050 and TM 9-8000 are works of the United States Navy and Army approved for public release, and no question or example is copied from them.
How it works
A free ASE S2 school bus diesel engines practice test of 55 questions, the number ASE scores on the real one. The page is unofficial and not affiliated with the National Institute for Automotive Service Excellence (ASE).
Read the full guide
Six content areas, 55 scored questions
ASE's School Bus study guide puts 15 questions each on general engine diagnosis and the fuel system. Air induction and exhaust, aftertreatment included, has 9. Lubrication and cooling has 7, in-chassis engine inspection and repair 6, and the starting system 3.
What each answer rests on
Engine noises, valve adjustment and camshaft work come from a 1992 Navy manual. Diesel exhaust fluid, particulate filters, common rail and unit injectors rest on Wikipedia. Fuel tank rules are from 49 CFR Part 393 and FMVSS No. 301.
School bus operations are excepted from 49 CFR Part 393 by 49 CFR 390.3(f)(1), so its fuel tank rules are asked as public yardsticks.
On aftertreatment, learn what each part is for: the particulate filter traps soot, and the SCR catalyst uses DEF to treat nitrogen oxides.
Questions and answers
How many questions are on the ASE S2 test?
Sixty-five in 90 minutes, of which 55 count toward the score, by ASE's 2026 School Bus study guide. Among the school bus tests only S2 and S6 run that long. The recertification test is 28 questions in 45 minutes.
Are these real ASE S2 test questions?
No. The 55 here are our own, written from ASE's S2 task list with a public source named under each. None is taken from the S2 test or from the ten sample questions in ASE's guide.
Is S2 required for Master School Bus Technician status?
Yes. ASE's School Bus study guide stars S2, S3, S4, S5 and S6 as the tests a technician needs for Master School Bus Technician status. S1 and S7 carry no star.
How is S2 different from the transit bus diesel test, H2?
H2 has 50 scored questions in seven areas, with cylinder head, engine block and charging system work. S2 has 55 in six, with one in-chassis repair area and the starting system alone. See the ASE H2 practice test.