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ASE H1 Study Guide

Unofficial practice for ASE's H1 Compressed Natural Gas Engines test: fuel cylinders, ignition and engine diagnosis on a transit bus.

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Methane makes up most of natural gas. Below what concentration in air is a methane mixture too lean to burn?

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Unofficial practice questions, not affiliated with the National Institute for Automotive Service Excellence (ASE). Bank checked October 14, 2026.

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All 50 questions with answers

General engine diagnosis 9

  • A fuel delivery fault makes a spark-ignited engine run lean. What does a lean mixture risk? Answer: Higher temperatures, which can form nitrogen oxides. Source: Wikipedia, Air–fuel ratio, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
  • A fuel line fitting on a CNG bus leaks in the open air. How does the escaped gas behave? Answer: It rises and disperses, being lighter than air. Source: Wikipedia, Compressed natural gas, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
  • A heavy natural gas engine of the stoichiometric type runs on which cycle, and how is its fuel ignited? Answer: The Otto cycle, by a spark. Source: Wikipedia, Natural gas vehicle, LNG fueled vehicles. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
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General engine diagnosis, continued

  • A technician smells gas near the roof cylinders of a bus. Where does the smell of natural gas come from? Answer: An odorant, a thiol, added to the gas. Source: Wikipedia, Thiol, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
  • An operator complains that a CNG bus has less range than a diesel bus with tanks of the same volume. How does the energy in a liter of CNG compare with a liter of diesel fuel? Answer: About 25 percent. Source: Wikipedia, Compressed natural gas, Energy density. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
  • Methane makes up most of natural gas. Below what concentration in air is a methane mixture too lean to burn? Answer: 4.4%. Source: Wikipedia, Flammability limit, Lower flammability limit. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
  • Spark knock is heard from a CNG engine under load. What is knocking in a spark-ignition engine? Answer: Mixture ignited by something other than the flame front from the plug. Source: Wikipedia, Engine knocking, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
  • Why is CNG less likely than gasoline to ignite on a hot surface such as an exhaust manifold? Answer: Its auto-ignition temperature is high, about 540 °C. Source: Wikipedia, Compressed natural gas, Advantages. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)
  • Why is natural gas less likely than gasoline to make an engine knock? Answer: It has a higher octane rating. Source: Wikipedia, Natural gas vehicle, Advantages over gasoline and diesel. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), A. General Engine Diagnosis (9 of 50 scored questions)

Cylinder head and valve train 4

  • A replacement spark plug has a threaded portion longer than the cylinder head is thick. What is the worst that can happen? Answer: The piston may strike the plug. Source: Wikipedia, Spark plug, Tip protrusion. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), B. Cylinder Head and Valve Train Diagnosis and Repair (4 of 50)
  • An engine has an aluminum cylinder head. What does that require at the valve seats? Answer: Steel valve seat inserts. Source: Wikipedia, Poppet valve, Materials and durability. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), B. Cylinder Head and Valve Train Diagnosis and Repair (4 of 50)
  • Spark plugs are specified by thread size, spark gap and sealing type. Which two sealing types are there? Answer: Taper seat or crush washer. Source: Wikipedia, Spark plug, Spark plug construction. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), B. Cylinder Head and Valve Train Diagnosis and Repair (4 of 50)
  • What is a valve stem oil seal for? Answer: To keep oil out of the intake manifold and combustion chamber. Source: Wikipedia, Poppet valve, Materials and durability. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), B. Cylinder Head and Valve Train Diagnosis and Repair (4 of 50)

Engine block 4

  • What do the core plugs of an engine block close? Answer: Holes left by the sand casting cores. Source: Wikipedia, Core plug, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), C. Engine Block Diagnosis, Repair, and Overhaul (4 of 50)
  • What is a dry cylinder liner? Answer: A liner surrounded by the block, out of contact with the coolant. Source: Wikipedia, Cylinder (engine), Cylinder liners/sleeves. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), C. Engine Block Diagnosis, Repair, and Overhaul (4 of 50)
  • Where is a harmonic damper fitted, and what does it counter? Answer: On the free end of the crankshaft, against torsional vibration. Source: Wikipedia, Harmonic damper, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), C. Engine Block Diagnosis, Repair, and Overhaul (4 of 50)
  • Which type of bearing usually carries a crankshaft at its main journals? Answer: A plain bearing held by the block and a bearing cap. Source: Wikipedia, Main bearing, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), C. Engine Block Diagnosis, Repair, and Overhaul (4 of 50)

Lubrication and cooling systems 4

  • Antifreeze lowers the freezing point of water. What else do common antifreezes do to the coolant? Answer: Raise its boiling point but lower its heat capacity. Source: Wikipedia, Antifreeze, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), D. Lubrication and Cooling Systems Diagnosis and Repair (4 of 50)
  • What does the anti-drainback valve of an oil filter do? Answer: Holds oil in the filter after the engine is shut down. Source: Wikipedia, Oil filter, Pressure relief valves. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), D. Lubrication and Cooling Systems Diagnosis and Repair (4 of 50)
  • Which pump types are commonly used as engine oil pumps? Answer: Gear, trochoid and vane pumps. Source: Wikipedia, Oil pump (internal combustion engine), Pumps. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), D. Lubrication and Cooling Systems Diagnosis and Repair (4 of 50)
  • Why can lubricating oil last longer in a CNG engine than in a gasoline engine? Answer: CNG does not contaminate and dilute the crankcase oil. Source: Wikipedia, Compressed natural gas, Advantages. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), D. Lubrication and Cooling Systems Diagnosis and Repair (4 of 50)

Air induction and exhaust systems 7

  • A pleated-paper air filter is correctly sized for the engine. When does it start to restrict airflow noticeably? Answer: Once it is significantly clogged with dirt. Source: Wikipedia, Air filter, Paper. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), E. Air Induction and Exhaust Systems Diagnosis and Repair (7 of 50)
  • A spark-ignited CNG engine has a throttle valve in its intake. What happens in the intake manifold when the throttle is partly closed? Answer: Pressure falls below ambient. Source: Wikipedia, Throttle, Internal combustion engines. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), E. Air Induction and Exhaust Systems Diagnosis and Repair (7 of 50)
  • In a spark-ignition engine, what side benefit comes from recirculating exhaust gas through an external EGR valve? Answer: Better efficiency, because a wider throttle opening cuts pumping losses. Source: Wikipedia, Exhaust gas recirculation, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), E. Air Induction and Exhaust Systems Diagnosis and Repair (7 of 50)
  • On a pneumatically controlled wastegate, what moves the actuator against its spring? Answer: Boost pressure from the intake. Source: Wikipedia, Wastegate, Pneumatic. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), E. Air Induction and Exhaust Systems Diagnosis and Repair (7 of 50)
  • What did building a heater into the ceramic of an oxygen sensor achieve? Answer: A sensor that starts sooner and responds faster. Source: Wikipedia, Oxygen sensor, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), E. Air Induction and Exhaust Systems Diagnosis and Repair (7 of 50)
  • What usually makes an unheated oxygen sensor fail? Answer: Soot building up on the ceramic element. Source: Wikipedia, Oxygen sensor, Sensor failures. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), E. Air Induction and Exhaust Systems Diagnosis and Repair (7 of 50)
  • Where does an internal wastegate send the exhaust it diverts? Answer: Around the turbine, into the downstream exhaust. Source: Wikipedia, Wastegate, Internal. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), E. Air Induction and Exhaust Systems Diagnosis and Repair (7 of 50)

CNG fuel system 14

  • A cylinder label reads "Do Not Use After" with a month and year. What does that date mark under FMVSS No. 304? Answer: The end of the manufacturer's recommended service life. Source: 49 CFR 571.304 (FMVSS No. 304), S7.4(h). Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • A fuel shutoff solenoid valve is described as normally closed. What does that mean? Answer: It is closed whenever the solenoid is not energized. Source: Wikipedia, Solenoid valve, Operation. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • A manual fuel shutoff is a quarter-turn ball valve. By convention, how does the handle sit when the valve is closed? Answer: Perpendicular to the flow. Source: Wikipedia, Ball valve, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • A rupture disc is one kind of pressure relief device. What happens after it has burst? Answer: It does not reseal and has to be replaced. Source: Wikipedia, Rupture disc, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • About what pressure is CNG stored at in vehicle cylinders? Answer: 2,900 to 3,600 psi. Source: Wikipedia, Compressed natural gas, lead section. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • An engine has electronic throttle control. What links the accelerator pedal to the throttle valve? Answer: No mechanical linkage: software drives the valve through an electric motor. Source: Wikipedia, Electronic throttle control, Failure modes. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • Besides the periodic inspection, after which events does the FMVSS No. 304 label say a CNG container should be visually inspected? Answer: A motor vehicle accident or a fire. Source: 49 CFR 571.304 (FMVSS No. 304), S7.4(g). Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • In the definitions of FMVSS No. 303, what is a dedicated CNG vehicle? Answer: A vehicle with one fuel system, designed to run on CNG. Source: 49 CFR 571.303 (FMVSS No. 303), S4, Definitions. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • In the definitions of FMVSS No. 303, where does the high pressure portion of a CNG fuel system end? Answer: At the first pressure regulator, which is not included. Source: 49 CFR 571.303 (FMVSS No. 303), S4, Definitions. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • The label of a CNG fuel container reads "Type 4". What does that designation mean under FMVSS No. 304? Answer: A non-metallic liner, full wrapped with resin impregnated filament. Source: 49 CFR 571.304 (FMVSS No. 304), S5.1.4. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • Under FMVSS No. 304, how high must the burst pressure of a composite (Type 2, 3 or 4) CNG container be? Answer: Not less than 2.25 times the service pressure. Source: 49 CFR 571.304 (FMVSS No. 304), S7.2.2. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • What does the symbol DOT on a CNG fuel container label certify under FMVSS No. 304? Answer: That the container manufacturer certifies it meets every requirement of the standard. Source: 49 CFR 571.304 (FMVSS No. 304), S7.4(d) and (f). Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • What is the job of the pressure regulator on a vehicle that runs on compressed gas? Answer: To reduce the stored gas pressure to operating pressure. Source: Wikipedia, Pressure regulator, Gas-powered vehicles. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)
  • Which FMVSS No. 304 container type is a metal container with no composite reinforcement? Answer: Type 1. Source: 49 CFR 571.304 (FMVSS No. 304), S5.1.1 and S7.4(e). Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), F. Fuel System Inspection, Diagnosis, and Repair (14 of 50)

Starting and charging systems 4

  • A flooded lead-acid battery is stored discharged for months. What happens to its electrolyte? Answer: The heavier acid settles and the electrolyte stratifies. Source: Wikipedia, Lead–acid battery, Absorbent glass mat. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), G. Starting and Charging System Diagnosis and Repair (4 of 50)
  • An alternator output test shows a charging voltage that is far too high. What does overcharging do inside a flooded lead-acid battery? Answer: It splits water into oxygen and hydrogen gas. Source: Wikipedia, Lead–acid battery, Charging. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), G. Starting and Charging System Diagnosis and Repair (4 of 50)
  • With the engine running, where does the voltage regulator hold the alternator output of a 12-volt system? Answer: Between 13.5 and 14.5 volts. Source: Wikipedia, Automotive battery, Gasoline and diesel engine. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), G. Starting and Charging System Diagnosis and Repair (4 of 50)
  • With the ignition on and the engine stopped, the charge warning indicator is lit. Why? Answer: Its circuit is grounded through the regulator until the alternator generates power. Source: Wikipedia, Alternator (automotive), Field regulation. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), G. Starting and Charging System Diagnosis and Repair (4 of 50)

Ignition system 4

  • How do the two windings of an ignition coil differ? Answer: The primary has few turns of heavy wire; the secondary has thousands of turns of fine wire. Source: Wikipedia, Ignition coil, Design. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), H. Ignition System Inspection, Diagnosis, and Repair (4 of 50)
  • How is the gap of a spark plug adjusted to the specification for the engine? Answer: By bending the ground electrode slightly. Source: Wikipedia, Spark plug, Spark plug gap. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), H. Ignition System Inspection, Diagnosis, and Repair (4 of 50)
  • One coil fails on an engine with coil-on-plug ignition. What is the service advantage of that design? Answer: Only the failed coil is replaced. Source: Wikipedia, Ignition coil, Distributorless systems. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), H. Ignition System Inspection, Diagnosis, and Repair (4 of 50)
  • Two spark plugs differ only in the length of the insulator tip. Which one runs hotter? Answer: The one with the longer insulator tip. Source: Wikipedia, Spark plug, Heat range. Outline: ASE Transit Bus Study Guide (copyright 2026), Compressed Natural Gas (CNG) Engines (Test H1), H. Ignition System Inspection, Diagnosis, and Repair (4 of 50)

How this list was compiled: Our own questions, written to the eight content areas of the Compressed Natural Gas (CNG) Engines (Test H1) test specifications and task list in the ASE Transit Bus Study Guide (copyright 2026, https://ase.com/wp-content/uploads/ASE_TransitBus_Study_Guide.pdf, opened 2026-10-14): general engine diagnosis 9 of 50 scored questions, cylinder head and valve train 4, engine block 4, lubrication and cooling 4, air induction and exhaust 7, fuel system 14, starting and charging 4, ignition 4. The bank holds the same 9, 4, 4, 4, 7, 14, 4 and 4. Every fact was read in the public text the question names: 49 CFR 571.304 (FMVSS No. 304, CNG fuel container integrity) and 49 CFR 571.303 (FMVSS No. 303) at ecfr.gov, and Wikipedia articles on compressed natural gas, natural gas vehicles, spark plugs, ignition coils, oxygen sensors, wastegates, catalytic converters, valves, regulators, batteries and engine parts (texts fetched 2026-10-13). The sample questions printed in the study guide were not used. Licence: Questions, choices and explanations are original work of this site.

How it works

This ASE H1 study guide is 50 practice questions on compressed natural gas engines for transit bus technicians, spread like the real test. It is unofficial and not affiliated with the National Institute for Automotive Service Excellence (ASE).

Read the full guide

What H1 covers: eight content areas

ASE's Transit Bus study guide gives the fuel system 14 of the 50 scored questions and general engine diagnosis 9. Air induction and exhaust has 7. Cylinder head, engine block, lubrication and cooling, starting and charging, and ignition have 4 each.

Where the CNG answers come from

Cylinder types, label lines and burst pressure come from FMVSS No. 304, the federal standard for CNG fuel containers. Gas properties, regulators, valves, spark plugs, coils, oxygen sensors and wastegates rest on Wikipedia.

  • Fuel is 28 percent of H1. Learn the four cylinder types and what each line of the cylinder label says.
  • H1 has an ignition content area: spark plugs, coils and sensors. The diesel test, H2, has none.

Questions and answers

Is there an official ASE H1 study guide?

Yes. ASE publishes a free Transit Bus Study Guide for tests H1 to H8. Its H1 pages hold the test specifications, the task list and six sample questions.

How many questions are on the ASE H1 test?

ASE's 2026 Transit Bus study guide lists 60 questions in 75 minutes: 50 scored and 10 that are not counted. The recertification test is 25 questions in 30 minutes.

Are these real ASE H1 questions?

No. Each one was written here from the H1 task list and a public source that the question names. None is taken from the H1 test or from the sample questions ASE prints for it.

Do I need H1 to be a Master Transit Bus Technician?

Either H1 or H2 counts. The study guide requires H3 through H8 plus one of the two engine tests, compressed natural gas (H1) or diesel (H2), for Master Transit Bus Technician status.

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