This free ASE A9 study guide teaches to the certification test — every content area the National Institute for Automotive Service Excellence tests, organized the way the exam is built.[1]The A9 test certifies that you can diagnose and repair the diesel engines found in light-duty trucks and cars: verifying a customer’s concern, running the right diagnostic test, and servicing the cylinder head, block, lubrication and cooling, air and exhaust, and the high-pressure fuel system.
The computer-based test has 60 questions (50 scored, 10 unscored research items) and 1 hour 15 minutes of testing time, spread across six content areas.[2] It is hands-on: questions are written by working technicians and focus on practical diagnosis, often using the format. This guide is interactive, not a wall of text — each area has a built-in checkpoint quiz, hover-able glossary terms, worked diagnostic scenarios, and concept questions.
Read this guide area by area, test yourself at each checkpoint, then round out your free A9 prep with our practice questions and flashcards.
ASE A9 is one of the 29 ASE certifications — explore our ASE study guides to compare and prep across the whole family.
ASE A9 Exam Snapshot
| Detail | ASE A9 Light Vehicle Diesel Engines |
|---|---|
| Questions | 60 administered (50 scored + 10 unscored research) |
| Time | 1 hour 15 minutes of testing |
| Format | Multiple choice, computer-based by appointment (Prometric) |
| Content areas | 6 (Fuel System and Air Induction & Exhaust carry the most questions) |
| Passing score | Scaled score; standard set per test by an expert panel (no fixed %) |
| Experience | ~2 years relevant work experience (or 1 year + 2-year degree) |
| Cost | 34 registration fee per order (fees can change) |
| Certification cycle | Valid 5 years; recertify via the A9 recert test or ASE Renewal App |
| Certifying body | ASE (National Institute for Automotive Service Excellence) |
The Fuel System and Air Induction & Exhaust areas together are about 60% of the scored test — diesel certification lives in fuel injection and aftertreatment, so master those first.
Because the Fuel System and Air Induction & Exhaust areas together are about 60% of the scored test, diesel fuel injection and aftertreatment matter most — but you still need solid diagnostic fundamentals across all six areas.[1] Here is the official distribution of the 50 scored questions:
This guide teaches all six content areas as six study modules. Before the areas, it helps to see what makes a diesel a diesel — compression ignition:
The defining difference from gasoline: a diesel injects fuel into already-compressed, super-hot air and lets it self-ignite. There is no spark plug — glow plugs only preheat the chamber to help cold starts.
- 1 · IntakePiston moves down; the intake valve opens and the cylinder draws in AIR ONLY (no fuel yet). A diesel has no throttle plate, so it pulls a full charge of air.
- 2 · CompressionBoth valves close; the piston compresses the air to roughly 15:1–17:1 on a modern light-duty diesel, heating it to about 540°C (1000°F) — hot enough to ignite fuel.
- 3 · Power (Combustion)Near top dead center the injector sprays a fine mist of fuel into the hot, compressed air; it self-ignites (compression ignition) and the expanding gases drive the piston down.
- 4 · ExhaustThe exhaust valve opens; the piston pushes the spent gases out toward the turbo and aftertreatment. Then the cycle repeats.
Gasoline: spark ignites a fuel-air mix. Diesel: high compression heats AIR, then injected fuel self-ignites. That is why diesels run high compression and no throttle.
1 · General Engine Diagnosis
About 22% of the scored test (11 questions). This is the diagnostic foundation: verifying the customer concern, reading symptoms, and running the right test to isolate a cause before you start replacing parts.[1]
On a diesel, black = fuel/air (over-fueling or low boost), blue = oil, white = unburned fuel or coolant. Smoke color is one of the fastest first clues in General Engine Diagnosis.
Compression Ignition & How Diesels Differ
The defining trait of a diesel is : it compresses air to a high (roughly 15:1–17:1 on modern light-duty engines), heating it enough that injected fuel self-ignites — no spark plug. only preheat the chamber to help cold starts. A diesel also has no throttle plate, so it always draws a full charge of air.
Compression Tests & Cold-Start (Glow Plugs)
A on a diesel must begin by disabling fuel injection, or the engine will try to start. A healthy light-duty cylinder reads about 300–450 psi — far higher than a gasoline engine. As with gasoline, a separates rings from valves.
| Test / symptom | What it tells you |
|---|---|
| Cranking compression test | Disable fuel first. Healthy light-duty diesel ≈ 300–450 psi; cylinders within ~10% of each other |
| Wet compression test | Reading rises with oil added = worn rings/bore; stays low = valves or head gasket |
| Glow plug resistance | Good ≈ 1 ohm; open (infinite) = burned-out element. All good but no preheat = check the relay/module |
| Cylinder balance test (scan tool) | Compares each cylinder's contribution to isolate a weak/misfiring cylinder |
| Fuel rail pressure (cranking) | Low rail pressure on a crank-no-start points to fuel supply, pump, or pressure control |
Smoke, Noises & Scan-Tool Diagnosis
Smoke color is a fast first clue (see the guide above): black = over-fueling or low air, blue = oil burning, white = unburned fuel or coolant. — a metallic rattle — comes from a too-rapid combustion pressure rise, often from mistimed injection, faulty injectors, or low- fuel. A scan-tool pinpoints a weak cylinder.
Checkpoint · Content Area 1 · General Engine Diagnosis
Question 1 of 10
Which of the following is a common cause of "diesel knock" or "diesel rattle" noise in a running diesel engine?
2 · Cylinder Head & Valve Train Diagnosis & Repair
About 12% of the scored test (6 questions). The seals the top of the cylinders and houses the valves, seats, guides, injectors, and camshaft. This area is about inspecting, machining, and reassembling the head and valve train correctly under diesel-level loads.[1]
Cylinder Head, Gasket & Liners
Diesel heads endure very high pressure and heat. A warped or cracked head causes head-gasket failure — letting coolant into a cylinder (white smoke, coolant loss) or combustion gases into the cooling system.
Check head flatness with a straightedge and feeler gauge and machine it flat if needed. Many diesels use replaceable , whose protrusion and sealing must be verified on assembly.
Valves, Guides, Seats & Seals
Valves must seal tightly against their seats. A burned or pitted valve, a worn , or a failed all cause compression loss or oil burning — worn guides and bad seals are a classic source of blue smoke.
| Fault | Symptom |
|---|---|
| Warped or cracked head | Coolant/oil leaks, overheating, compression loss |
| Blown head gasket | White smoke, coolant loss, milky oil, bubbles in coolant |
| Burned/leaking valve | Low compression in one cylinder; weak cylinder on a balance test |
| Worn valve guide / bad stem seal | Oil into the chamber → blue smoke and oil consumption |
| Worn or receded valve seat | Poor sealing, compression loss, lash change |
Camshaft, Lifters & Valve Lash
The camshaft opens the valves in time with the crankshaft. Correct is essential: too little holds a valve open and burns it; too much causes a tap and accelerates wear. Many diesels require periodic mechanical lash adjustment, while engines with hydraulic lifters take it up automatically.
Checkpoint · Content Area 2 · Cylinder Head & Valve Train
Question 1 of 10
In a diesel engine, what component is responsible for opening and closing the intake and exhaust valves?
3 · Engine Block Diagnosis & Repair
About 12% of the scored test (6 questions) — high-precision machining work. The block holds the cylinders, crankshaft, pistons, and bearings under diesel-level loads. This area is about measuring wear and machining or fitting parts to spec.[1]
Block, Liners, Bores & Machining
Diesel blocks are usually heavy cast iron for strength and heat dissipation, and many use replaceable . Worn bores are restored by boring (to size), (the crosshatch finish), (flattening the deck), and (truing the main bores). Cracks are found with magnetic-particle, dye-penetrant, or sonic (ultrasonic) testing.
Crankshaft, Bearings, Pistons & Rings
Main and rod bearing clearances are checked with or a micrometer — too much clearance causes a deep bottom-end knock and low oil pressure. Diesel pistons take enormous load; rings seal compression and control oil, and worn rings cause blue smoke and low compression. Check ring end gap in the bore when fitting new rings.
Checkpoint · Content Area 3 · Engine Block
Question 1 of 10
Which type of engine block material is commonly used in diesel engines due to its excellent durability and heat dissipation properties?
4 · Lubrication & Cooling Systems Diagnosis & Repair
About 14% of the scored test (7 questions). Diesel oil works hard — lubricating heavily loaded bearings and carrying away heat and soot — and the cooling system keeps the engine (and its aftertreatment) in the right temperature range.[1]
Oil, Pump & Lubrication
The circulates pressurized oil through the bearings and galleries. Low oil pressurecan come from low oil level, too-thin or fuel-diluted oil, a worn pump, a clogged pickup, or worn bearings. Diesel oil must meet the engine’s diesel-rated specification — it carries extra detergent/dispersant additives to hold soot in suspension.
Coolant, Thermostat, Pump & Radiator
A stuck closed causes overheating; stuck open keeps the engine too cool, hurting economy, heat, and aftertreatment performance. The water pump circulates coolant and the radiator cap raises the boiling point. Many diesels add an oil cooler and an EGR cooler — a leaking EGR cooler can put coolant into the intake and exhaust.
| Fault | Result |
|---|---|
| Low oil pressure | Low level, thin/fuel-diluted oil, worn pump/pickup, or worn bearings |
| Thermostat stuck closed | Engine overheats — no flow to the radiator |
| Thermostat stuck open | Runs too cool — poor economy, weak heat, poor regen/emissions |
| Failed water pump | No coolant circulation; overheating; leak at weep hole |
| Leaking EGR cooler / head gasket | Coolant into intake or cylinder; white smoke; coolant loss |
Checkpoint · Content Area 4 · Lubrication & Cooling Systems
Question 1 of 10
What is the primary function of the engine oil pump in a diesel engine?
5 · Air Induction & Exhaust Systems Diagnosis & Repair
About 28% of the scored test (14 questions) — the second-largest area. Diesels depend on forced induction and on heavy aftertreatment to make power cleanly. Expect questions on the turbo, intercooler, EGR, and the DOC/DPF/SCR train.[1][5]
Air Filter, Turbo & Intercooler
Follow the charge-air path: clean air in, boosted by the , cooled and densified by the , then distributed to the cylinders.
- Air filter— Draws in and filters outside air — clean air in is essential or boost and combustion suffer.
- Turbocharger compressor— Exhaust gas spins the turbine, which drives the compressor to pressurize (boost) the intake air. Many diesels use a variable-geometry turbo (VGT).
- Intercooler (charge-air cooler)— Cools the compressed, heated air so it is denser — more oxygen per cylinder for more complete, powerful combustion.
- Intake manifold → cylinders— The cool, dense, pressurized air is distributed to the cylinders for combustion.
A leak in the charge-air (boost) plumbing hisses under load and drops boost; a wastegate or VGT actuator controls how much boost the turbo makes.
Many light-duty diesels use a the ECM controls; others use a to cap . A charge-air (boost) leak hisses under load and reduces power.
EGR, DOC, DPF & SCR/DEF Aftertreatment
Diesel emissions control is a big part of A9. cools combustion to cut NOx. Downstream, the aftertreatment train cleans the exhaust in order:
- DOC — Diesel Oxidation Catalyst— Oxidizes carbon monoxide (CO) and unburned hydrocarbons (HC) into CO₂ and water; also generates heat to help regenerate the DPF.
- DPF — Diesel Particulate Filter— Traps soot (particulate matter). It periodically REGENERATES — burning off trapped soot at high temperature — passively, actively, or via a forced/service regen.
- DEF injection (urea)— Diesel Exhaust Fluid (a 32.5% urea / water solution) is sprayed into the hot exhaust ahead of the SCR, where it breaks down into ammonia.
- SCR — Selective Catalytic Reduction— Uses the ammonia from DEF to convert nitrogen oxides (NOx) into harmless nitrogen and water.
Memory aid: DOC cleans CO & HC, DPF catches soot, SCR + DEF kills NOx. EGR also lowers NOx by cooling combustion — together they meet diesel emissions limits.
The cleans CO and hydrocarbons, the traps soot and , and uses ammonia from to destroy NOx.
Many ASE A9 items give two technicians’ statements and ask who is right. Judge each statement separately as true or false, then map to the answer:
The trap is letting a true statement A make you ignore a false statement B. Evaluate both before you choose.
Checkpoint · Content Area 5 · Air Induction & Exhaust Systems
Question 1 of 10
What is the purpose of a diesel particulate filter (DPF) in the exhaust system of a diesel vehicle?
6 · Fuel System Diagnosis & Repair
About 32% of the scored test (16 questions) — the single biggest area. Modern light-duty diesels use high-pressure injection with electronically controlled injectors. Master the fuel path, the components, and the diagnostic logic — this area carries the most weight on A9.[1]
Common Rail & High-Pressure Delivery
Follow the fuel path from tank to cylinder:
- 1 · Fuel tank → lift (transfer) pumpA low-pressure lift pump pulls fuel from the tank and pushes it forward to the filter and high-pressure pump.
- 2 · Fuel filter / water separatorRemoves dirt and — critically on a diesel — water, which can destroy injectors. Many systems have a primary and a secondary filter.
- 3 · High-pressure pumpA cam-driven pump raises fuel to extreme pressure — often 20,000–30,000+ psi (about 1,500–2,000+ bar) — feeding the common rail.
- 4 · Common rail (accumulator)A shared high-pressure rail holds fuel ready for all injectors; a rail pressure sensor and pressure-control valve regulate it.
- 5 · Electronic injectorsThe ECM commands each solenoid- or piezo-actuated injector to fire precisely — often multiple injections per stroke (pilot, main, post).
- 6 · Return lineExcess fuel and injector leak-off return to the tank, helping cool the system.
Common rail separates pressure generation (the pump) from injection timing (the ECM-controlled injectors), giving precise, multi-event injection for power, economy, and low emissions.
A feeds the , which charges the to extreme pressure (often 20,000–30,000+ psi). The ECM then fires each precisely — often with a to soften combustion and reduce knock.
Injectors, Filtering & Fuel Quality
Because injection runs at extreme pressure with fine tolerances, clean, water-free fuel is critical. The fuel filter removes particulate and the removes water, which can destroy the pump and injectors.
A restricted fuel filter limits delivery — causing low power, often with little or no smoke because the mixture stays lean. On a crank-no-start, check fuel rail pressure first.
| Component / fault | Function or symptom |
|---|---|
| Lift (transfer) pump | Low-pressure supply from tank to the high-pressure pump |
| High-pressure pump | Raises fuel to extreme pressure for the common rail |
| Common rail + pressure sensor | Shared accumulator; sensor + control valve regulate rail pressure |
| Electronic injectors | ECM-fired precise injection; multiple events (pilot/main/post) per stroke |
| Fuel filter / water separator | Remove dirt and water; restricted filter = low power, little smoke |
Checkpoint · Content Area 6 · Fuel System
Question 1 of 10
What type of fuel is commonly used in diesel engines?
How to Use This Study Guide
A study guide is a map, not the whole territory — use it alongside hands-on shop experience and our free tools. Because A9 weights the Fuel System and Air Induction & Exhaust areas most heavily, spend the most time on common-rail injection, the turbo/intercooler, and the EGR/DOC/DPF/SCR train. Read every item carefully, judging each statement on its own before you answer.
- 1
Read a content area here
Work through one area at a time — lead with Fuel System and Air Induction & Exhaust, the two biggest areas.
- 2
Take the checkpoint
The quick check at the end of each area exposes what didn't stick.
- 3
Drill the gaps
Send your weak area straight into the free practice questions and flashcards.
- 4
Test under exam conditions
Take full, timed practice sets and review every miss — especially the diagnostic reasoning.
ASE A9 Concept Questions
Common diesel-engine concepts the A9 test actually measures — at least one per content area. Tap any card for a short, exam-ready answer backed by an authoritative source, then test yourself on them as flashcards.
ASE A9 Glossary
Quick definitions for the terms you’ll see most across the ASE A9 Light Vehicle Diesel Engines test:
- Align honing
- Machining the main bearing bores back into true alignment so the crankshaft turns freely and bearings wear evenly.
- ASE A9
- The ASE Light Vehicle Diesel Engines certification test, part of the Automobile and Light Truck (A-series) program from the National Institute for Automotive Service Excellence. It certifies a technician's knowledge of diagnosing and repairing diesel engines in light-duty vehicles.
- Boost pressure
- The intake-air pressure produced by the turbocharger above atmospheric. A charge-air (boost) leak hisses under load and reduces power.
- Cetane number
- A measure of how readily diesel fuel ignites under compression — the diesel counterpart to gasoline's octane. Higher cetane ignites more easily and reduces knock and cold-start trouble.
- Common rail
- A high-pressure fuel-injection system with a shared rail (accumulator) that holds fuel at extreme pressure for all injectors, while the ECM controls injection timing and quantity electronically.
- Compression ignition
- The diesel ignition principle: air is compressed so highly that it becomes hot enough to ignite injected fuel on its own, with no spark plug.
- Compression ratio
- The ratio of cylinder volume at bottom dead center to top dead center. Modern light-duty diesels run roughly 15:1 to 17:1 — much higher than gasoline — to create the heat for compression ignition.
- Cranking compression test
- A test of the pressure each cylinder builds while cranking; on a diesel, fuel injection must first be disabled so the engine only cranks. A healthy light-duty diesel reads about 300–450 psi per cylinder.
- Cylinder balance test
- A scan-tool test that compares each cylinder's contribution to crankshaft acceleration to isolate a weak or misfiring cylinder.
- Cylinder head
- The casting that seals the top of the cylinders and houses the valves, seats, guides, injectors, and (on overhead-cam engines) the camshaft.
- Cylinder liner (sleeve)
- A replaceable cylinder bore insert used in many diesels so worn bores can be re-serviced. Wet liners contact the coolant directly; dry liners press into the block.
- Decking
- Machining the block's top (deck) surface flat and smooth so the head gasket seals correctly.
- DEF (Diesel Exhaust Fluid)
- A 32.5% urea and water solution sprayed into the hot exhaust ahead of the SCR catalyst, where it breaks down into the ammonia used to reduce NOx.
- Diesel knock
- The metallic rattle from the rapid combustion pressure rise when injected fuel ignites. Worsened by over-advanced or mistimed injection, low-cetane fuel, or faulty injectors.
- DOC (Diesel Oxidation Catalyst)
- An aftertreatment catalyst that oxidizes carbon monoxide and unburned hydrocarbons into carbon dioxide and water, and helps heat the DPF.
- DPF (Diesel Particulate Filter)
- A filter that traps soot (particulate matter) from the exhaust and periodically regenerates by burning the soot off at high temperature.
- DPF regeneration
- The process of burning accumulated soot out of the DPF. It can be passive, active (the system adds heat), or a forced/service regen performed with a scan tool.
- EGR (Exhaust Gas Recirculation)
- A system that routes metered, often cooled exhaust gas back into the intake to lower peak combustion temperature and reduce oxides of nitrogen (NOx).
- Fuel injector
- The ECM-controlled (solenoid or piezo) valve that sprays a precise, finely atomized charge of fuel into the combustion chamber, often with multiple injection events per stroke.
- Glow plug
- An electric heating element that preheats the diesel combustion chamber (or pre-chamber) to help the engine start in cold weather. A good glow plug reads near 1 ohm; an open reading means it is burned out.
- High-pressure fuel pump
- The cam-driven pump that raises fuel to the extreme pressure (often 20,000–30,000+ psi) supplied to the common rail and injectors.
- Honing
- Machining the cylinder bore to a smooth finish with a crosshatch pattern that helps the rings seat and retains a film of oil.
- Intercooler (charge-air cooler)
- A heat exchanger that cools the compressed, heated intake air so it is denser — more oxygen per cylinder for more complete combustion.
- Lift (transfer) pump
- A low-pressure pump that moves fuel from the tank through the filters to the high-pressure pump.
- Oil pump
- The pump that circulates pressurized oil through the bearings, lifters, and galleries; a worn pump, thin/diluted oil, a clogged pickup, or worn bearings can cause low oil pressure.
- Pilot injection
- A small early injection that begins combustion gently before the main injection, softening the pressure rise to reduce diesel knock and noise.
- Plastigage
- A calibrated plastic strip crushed between a bearing and journal to measure oil clearance by comparing the flattened width to a chart.
- SCR (Selective Catalytic Reduction)
- An aftertreatment system that uses ammonia (from injected DEF) to convert oxides of nitrogen (NOx) into harmless nitrogen and water.
- Technician A / Technician B
- The signature ASE question format presenting two statements; you decide whether A only, B only, both, or neither is correct.
- Thermostat
- A temperature-controlled valve that blocks coolant flow to the radiator until the engine warms, then opens to regulate operating temperature. Stuck closed = overheating; stuck open = runs too cool.
- Turbocharger
- An exhaust-driven compressor that pressurizes (boosts) the intake air so more air enters the cylinders for more power. Many light-duty diesels use a variable-geometry turbo (VGT).
- Valve guide
- The bore in the cylinder head that supports and aligns the valve stem; wear lets oil into the chamber and the valve seal poorly.
- Valve lash
- The small valve-train clearance that allows for thermal expansion. Too little burns valves and loses compression; too much causes a tapping noise and wear. Many diesels need periodic mechanical adjustment.
- Valve stem seal
- A seal that keeps oil from running down the valve stem into the combustion chamber or exhaust port; failure causes blue smoke and oil consumption.
- Variable-geometry turbo (VGT)
- A turbocharger with movable vanes the ECM adjusts to control boost across the RPM range, improving low-speed response and reducing lag.
- Wastegate
- A valve that bleeds exhaust around the turbine to limit and control maximum boost pressure.
- Water separator
- A filter element that removes water from diesel fuel; water must be kept out because it corrodes and destroys high-pressure pumps and injectors.
- Wet compression test
- A compression test repeated after adding a little oil to the cylinder. If the reading rises, the rings or cylinder walls are worn; if it stays low, the valves or head gasket leak.
Free ASE A9 Study Materials & Resources
Everything you need to prepare for the ASE A9 test is free here — no paywall, no sign-up. This guide is the foundation; pair it with the rest of our free A9 study materials for active recall, timed practice, and last-minute review:
- ASE A9 Practice Test — exam-style questions across all six content areas, with explanations.
- ASE A9 Flashcards — active-recall decks for the components, procedures, and specs you must know cold.
ASE A9 Study Guide FAQ
The ASE A9 Light Vehicle Diesel Engines test has 60 multiple-choice questions and 1 hour and 15 minutes of testing time. Of the 60, 50 are scored and 10 are unscored research questions ASE is trying out for future tests; they are not identified, so answer every question.
ASE A9 covers six content areas: General Engine Diagnosis, Cylinder Head and Valve Train Diagnosis and Repair, Engine Block Diagnosis and Repair, Lubrication and Cooling Systems Diagnosis and Repair, Air Induction and Exhaust Systems Diagnosis and Repair, and Fuel System Diagnosis and Repair. The Fuel System and Air Induction and Exhaust areas carry the most questions.
There is no fixed percentage. Raw scores are converted to a scaled score, and a panel of subject-matter experts sets the passing standard for each test form (a criterion-referenced cut score) so the bar stays consistent even as question difficulty varies. Your overall result, not any single content area, decides pass or fail.
The A9 test is computer-based and delivered by appointment at a Prometric testing center. You register through your myASE account, schedule the appointment, and typically have 90 days from purchase to test. If you fail, you must wait before retaking and pay the test fee again.
ASE requires about two years of relevant hands-on work experience, or one year of experience plus a two-year degree in automotive repair, to earn the certificate. You may pass the test first; ASE holds your result and issues the certification once you document the required experience.
ASE A9 certification is valid for five years. You recertify by passing the shorter current A9 recertification test, or, if you hold multiple A-series certifications, by using the ASE Renewal App, which delivers smaller sets of questions over time instead of one sit-down test.
A diesel uses compression ignition — it compresses air until it is hot enough to ignite injected fuel, with no spark plug. That drives the rest of the test: high compression ratios, glow plugs for cold starts, high-pressure common-rail fuel injection, turbocharging with intercooling, and diesel-specific aftertreatment (EGR, DOC, DPF, and SCR with DEF).
It is the signature ASE format: two technicians each make a statement, and you choose whether Technician A only is correct, Technician B only, both, or neither. Judge each statement separately as true or false, then pick the answer that matches — do not let a true statement A make you overlook a false statement B.
Work through the six content areas, spending the most time on the Fuel System and Air Induction and Exhaust areas, which carry the most questions. After each area, take the checkpoint quiz to find gaps, drill them with our free practice questions and flashcards, and revisit the diagrams and worked scenarios before test day.
Yes — the full guide, the checkpoints, the glossary, the practice questions, and the flashcards are 100% free, with no account required.
References
- 1.ASE (National Institute for Automotive Service Excellence). “A9 Light Vehicle Diesel Engines Certification Test.” ASE. ↑
- 2.ASE. “Automobile and Light Truck Certification Tests (A-Series).” ASE. ↑
- 3.ASE. “Dates, Fees & Test Times.” ASE. ↑
- 4.ASE. “myASE Account & Test Registration.” ASE. ↑
- 5.U.S. Environmental Protection Agency. “Diesel Vehicles, Air Pollution & Emission Controls.” U.S. EPA. ↑
Sources for the concept answers
Every answer in the ASE A9 concept questions above is drawn from an authoritative primary source:
Career Employer
Career Employer is the ultimate resource to help you get started working the job of your dreams. We cover topics from general career information, career searching, exam preparation with free study materials, career interviewing, and becoming successful in your career of choice.
All PostsCareer Employer’s Editorial Process
Here at Career Employer, we focus a lot on providing factually accurate information that is always up to date. We strive to provide correct information using strict editorial processes, article editing, and fact-checking for all of the information found on our website. We only utilize trustworthy and relevant resources. To find out more, make sure to read our full editorial process page here.