ASVAB Auto Information study guide

One part per row across twelve system tables — the job each part does and the one fact that separates it from its neighbor — the four-stroke cycle as one picture, the confusable pairs this subtest is built on, what each dash warning light means and how urgent it is, the tire sidewall code read piece by piece, and the one-sentence job for every part the bank asks about.

What this subtest actually asks of you

Auto Information is 10 of the 135 scored questions on the computer ASVAB — 7% of the scored test. Like Electronics Information, it is not one of the four AFQT subtests, so it does not move the number that decides whether you can enlist — but it does feed the line scores that decide which jobs you qualify for (how the ASVAB is scored). On the paper form Auto Information and Shop Information are reported as one combined score; the computer test asks them as two separate subtests of 10 questions each, and that is the split this site follows.

Almost all of it is recognition: which part does which job, and what happens when that part fails. There is nothing to calculate — every question on this subtest is a function question, phrased either "what does X do?" or "which part does Y?"

Two consequences:

Learn the systems, not a parts list. Every part belongs to a system, and the system tells you what the part does even when its name is new to you. A part whose job you can place is a question you can answer.
Learn each part with one sentence attached. "The radiator dumps coolant heat into the air." The wrong choices are almost always a real part from a neighboring system — the radiator offered for the thermostat's job — so knowing the boundary is worth more than knowing the definition.

The parts map: how to place a part you have not met

One table per system. When a question names a part you cannot place, ask which system it must belong to — then what that system has to accomplish for the engine to survive.

Lubrication

Keeps metal off metal inside the engine.

Question	What it does	The one fact to know
What does the engine oil do?	forms a film between every moving part it can reach, so metal never grinds on metal	it does not wear out the way tires do — it gets contaminated, which is why it is changed with the filter
What does the oil filter do?	traps the dirt and metal particles so they stop circulating through the engine	it is holding the dirt the old oil was carrying, so leaving it in place puts that dirt straight into the fresh oil
What does the oil pump do?	pushes the oil around the engine under pressure	a car can have a full sump and still lose pressure — pressure is the fault the dash light reports, and level is a different job
What does the dipstick do?	shows how much oil is in the engine	read it with the engine off and the car level: pull, wipe, reinsert it fully, then read
What does 5W-30 mean on an oil bottle?	the oil's thickness rating, written cold-first	the first number is the cold-weather flow rating, and a lower first number means oil that flows more easily when cold

Cooling

Carries heat away from the engine.

Question	What it does	The one fact to know
What does the radiator do?	passes the coolant's heat to the air flowing through it	it is only half a system on its own — a fan keeps that air moving whenever the car is stopped
What does coolant (antifreeze) do?	carries heat from the engine to the radiator	it does two jobs beyond carrying heat: it lowers the freezing point of the water in the cooling system, and it raises its boiling point
What does the water pump do?	circulates the coolant between the engine and the radiator	the coolant has to be moved, not merely present — no circulation means the heat never leaves the engine
What does the thermostat do?	controls where the coolant may flow, so the engine warms quickly and then holds its designed temperature	it is the one failure whose direction matters: stuck shut traps the heat and the temperature gauge climbs
What does the radiator cap do?	seals and pressurizes the cooling system	the pressure is the point — it is what lets the coolant run far above its normal boiling point without boiling

Fuel and air

Gets fuel and air into the engine.

Question	What it does	The one fact to know
What does the fuel pump do?	pushes fuel from the tank to the engine	it moves a liquid; the pump and the filter are the two parts between the tank and the injectors
What does the fuel filter do?	keeps dirt and rust out of the fuel before it reaches the engine	an injector's orifice is measured in microns, so the filter is what protects the part that cannot be cleaned out
What do the fuel injectors do?	spray a fine mist of fuel into the intake or the cylinder	the computer decides how much, the injector decides how finely — a clogged one delivers less than it was told to
What does the turbocharger do?	forces more air in so the engine can burn more fuel and make more power	it is driven by the exhaust, where a supercharger is driven by a belt off the engine — the boundary that pair is asked on
What does the air filter do?	cleans the air on its way in	less air in means less fuel burned, so a clogged one costs power and fuel mileage rather than breaking anything

Ignition

Lights the air-fuel mixture.

Question	What it does	The one fact to know
What does the spark plug do?	fires the mixture at the end of the compression stroke	a diesel has none: the heat of compression lights the fuel instead, which is the difference between the two engines
What does the ignition coil do?	steps the battery's 12 volts up to the thousands of volts the plug needs	it is the part to name when the question asks what raises the voltage
What does the distributor do?	sends the coil's high voltage to each spark plug in turn	it does it in the engine's firing order; a modern car does the same job with one coil per plug

Electrical

Supplies electrical energy and cranks the engine.

Question	What it does	The one fact to know
What does the battery do?	stores electrical energy to crank the engine and run the lights with the engine off	it is the part that stores; the alternator is the part that makes
What does the alternator do?	makes electricity while the engine is running	it does two jobs at once — runs the car's electrical system and puts back what the battery has spent
What does the starter motor do?	turns the engine over so it can fire	it draws a heavy current, which is why a slow crank with dim lights points at the battery first
What does the fuse do?	melts to open the circuit before too much current can damage the wiring	it melts because too much current flowed, not because the voltage rose — it is the cheap part that fails on purpose
What does the wiring do?	carries the current from the battery to the loads and back to ground	a circuit has to be a complete loop: a corroded ground stops the current as surely as a cut wire

Exhaust and emissions

Carries the burned gases out and cleans them. The three vapor and gas parts — the PCV valve, the EGR valve and the charcoal canister — have their own table further down.

Question	What it does	The one fact to know
What does the exhaust manifold do?	collects the burned gases from the cylinders	it is the first part in the chain, bolted straight to the engine
What does the catalytic converter do?	turns much of the harmful gas into less harmful gas	it can only clean a mixture it was designed for, which is why a dead oxygen sensor fails an emissions test before a probe reaches the tailpipe
What does the muffler do?	reduces the noise	it changes how the car sounds and nothing else — the gas leaving it is still poisonous
What does the tailpipe do?	releases the gases at the back of the car	it ends as far from the passenger compartment as the car allows, because what it carries is carbon monoxide — never leave an engine running in a closed garage

Sensors and computer

Measures what the engine is doing and corrects it.

Question	What it does	The one fact to know
What does the oxygen sensor do?	sits in the exhaust, after the burn, and reports how much oxygen is left over	the exhaust is the report card — the computer trims the next mixture from it, and with the sensor dead there is no feedback to trim from
What does the mass airflow sensor do?	sits in the intake, before the burn, and measures the air on its way in	more air needs more fuel, so it is what decides the amount added (older cars used a manifold absolute pressure sensor for the same job)
What does the engine computer do?	decides how much fuel to inject and when	it can only decide well if the sensors tell it what is happening — no input, no correct mixture
What does the OBD-II port do?	the socket a scan tool plugs into to read the stored fault codes	it sits under the dash near the driver's left knee, and a code names the circuit the computer is unhappy with, so the technician still tests it

Engine

Changes up-and-down motion into rotation.

Question	What it does	The one fact to know
What do the pistons do?	move up and down inside the cylinders	only the power stroke pushes them down; the other three cost the engine work, which the crankshaft and the flywheel pay for
What does the crankshaft do?	collects the pistons' up-and-down motion and hands it onward as rotation	every question about "what turns the pistons' motion into rotation" is a question about the crankshaft
What do the cylinders do?	the bores the pistons travel in	how many of them is a count, where the engine's liters are a size
What do the valves do?	open to let the mixture in and the burned gas out	both are shut through the compression stroke, and that seal is what makes the squeeze work
What does the camshaft do?	opens the valves at the right moment in the engine's cycle	it has to stay in step with the crankshaft — the timing belt is what keeps the two together
What does the timing belt do?	keeps the crankshaft and the camshaft turning in step	if it breaks, the valves and the pistons can meet, which is why it is replaced on schedule rather than on noise
What is engine displacement?	the total volume the pistons sweep as they travel	it is the number on the badge, 2.0 or 5.7 liters, and it is a size, not a count

Drivetrain

Turns rotation into wheels at the right speed.

Question	What it does	The one fact to know
What does the clutch do?	connects and disconnects the engine and the transmission in a manual car	it is why a manual can sit stopped, in gear, with the engine running
What does the torque converter do?	couples an automatic's engine to its transmission through fluid	fluid rather than friction is the whole difference from a clutch — and why an automatic has no clutch pedal
What does the transmission do?	supplies the gear ratios	it is what changes the trade between turning speed and turning force
What is a gear ratio?	the turns of a small driving gear for one turn of a larger driven gear: the small gear turns slowly but with more turning force, and driving the large gear into the small one gives speed back instead	slowly with force, or quickly with less — that trade is every gear question on this subtest
What does the differential do?	lets the two driven wheels turn at different speeds	it is not for changing the car's speed: in a turn the outer wheel covers more ground than the inner one
What do the drive shafts do?	carry the rotation from the transmission out to the wheels	which pair they run to is what makes a car front- or rear-wheel drive — that pair is the pair that pulls
What does the universal joint (U-joint) do?	lets a spinning shaft keep turning while the angle between its two ends changes	it is the driveshaft's joint, where the rear axle rises and falls as the car travels
What does the CV joint (constant-velocity joint) do?	drives a wheel through a large angle at a steady speed	a U-joint's shaft pulses in speed unless it is used in pairs, so a steered, driven wheel needs a CV joint instead

Brakes

Turns motion into heat and stops the car.

Question	What it does	The one fact to know
What does the master cylinder do?	turns the brake pedal's force into hydraulic pressure	pressure is the same throughout the fluid, which is why a light push on the pedal stops a heavy car
What does the brake fluid do?	carries that pressure to a caliper at each wheel	the one fluid on the car changed on time rather than mileage: it takes in water, water boils into vapor, and vapor compresses where liquid does not
What does the caliper do?	presses the brake pads onto the spinning disc	it is the part that squeezes; the pads are the part that wears
What do the brake pads do?	press against the disc to slow it	they are the friction material that wears out, and the cheapest part to replace before it becomes a disc
What do the discs do?	spin with the wheel and give the pads something to press against	the friction is between a stationary pad and a moving disc, which is why a warped one is felt through the pedal
What do the drum brakes do?	press curved brake shoes outward against the inside of a drum	a disc's mirror image: the shoes are the friction material, pressed out from inside a closed drum, so a drum shows no pads to inspect
What does anti-lock braking do?	pumps the brakes far faster than a driver can	the point is not a shorter stop on every surface: it is keeping the wheels from locking so you can still steer while braking hard

Steering and suspension

Keeps the wheels pointed and on the road.

Question	What it does	The one fact to know
What do the springs do?	hold the car up and absorb the bumps	they carry the car's weight, which is why a broken one sags a corner
What do the shock absorbers do?	control the springs' bounce	they are what keeps the tires in contact with the road after a bump rather than bouncing off it
What does the steering gear do?	turns the driver's steering input into a movement of the front wheels	it is the link between the wheel you turn and the wheels that point
What is wheel alignment?	sets the angles at which the front wheels point — toe, camber and caster	it is an adjustment to those angles, not a part that wears: a car out of alignment shows it as a pull or as tires worn unevenly, not as a noise

Tires

The only contact with the road. The sidewall size code is not one of these three — it is the table below.

Question	What it does	The one fact to know
What does tire tread do?	the grooved rubber that grips the road	the grooves are what channel water away, so worn tread is a wet-road fault before it is a dry-road one
Why does tire pressure matter?	how much air is in the tire	a contact patch the size of your hand carries the whole car: underinflated tires flex and hurt fuel mileage, overinflated ones wear the middle of the tread
Why rotate tires?	moves the tires between the car's positions	front and rear tires wear differently, so rotating them evens the wear out and all four reach the end of their life together

The sidewall is a code, not a size, and it is pure recall. Read P215/65R15 one piece at a time:

Question	What it says
What does P mean on a tire sidewall?	a passenger-car tire
What does 215 mean on a tire sidewall?	the tire's width in millimeters (its section width, tread shoulder to tread shoulder)
What does 65 mean on a tire sidewall?	the sidewall height as a percentage of that width — the aspect ratio, so a lower number is a shorter, stiffer sidewall
What does R mean on a tire sidewall?	radial construction
What does 15 mean on a tire sidewall?	the wheel it fits, in inches

Two traps. The first number is a width, not a height — the height is the second number, and it is a percentage of the first. And the units change across the code: millimeters first, inches last. So 65 does not mean 65 millimeters of sidewall; it means 65% of 215 mm.

That is every system on the car and the parts it uses — the warning lights and gauges have their own table in the dash section below. Placing a part in its sub-table is the whole method: if you can say which sub-table it lives in, you usually remove half the choices before you know anything else about the part.

The one cycle worth memorizing: the four strokes

Stroke order is the one sequence on this subtest worth knowing cold, and compression and power are the two that get mixed up. The whole sequence is intake, compression, power, exhaust — and it is easier to hold as four pictures than as four words.

1. Intake. The piston travels down and the intake valve is open, so air and fuel are drawn into the cylinder.

2. Compression. Both valves shut and the piston travels up, squeezes the air-fuel mixture into a much smaller space. Nothing burns before it has been squeezed — the squeeze is what makes the mixture hot enough to burn fast.

3. Power. The spark plug fires just as the piston finishes the compression stroke, and the burning gas pushes the piston down. Only the power stroke produces work; the other three strokes cost the engine work, which is what the crankshaft and the flywheel's momentum pay for.

4. Exhaust. The exhaust valve opens and the piston travels up again, pushing the burned gases toward the exhaust system.

Two details worth keeping:

The crankshaft is what converts motion. The pistons move up-and-down; the crankshaft collects that up-and-down motion and hands it onward as rotation. Every question about "what turns the pistons' motion into rotation" is a question about the crankshaft.
A diesel has no spark plug involved at all. In a diesel, the heat of compression ignites the fuel, so no spark plug is involved — the same four strokes, a different way of lighting the mixture. In a gasoline engine the ignition coil steps the battery's 12 volts up to the many thousands of volts the plug needs, which is why the coil is the part to name when the question asks what raises the voltage.

The four-step method for a part you cannot recall

1. Name the system. "That is a cooling system part" or "that is part of the ignition" — naming the row of the table usually removes two choices before you know anything about the part itself.

2. Say what the system has to achieve. A cooling system must carry heat away from the engine, so its parts circulate coolant, control the flow of it, or hand the heat to the air. A system's parts are its verbs: lubricates, supplies, cranks, cools, quiets.

3. Eliminate the swapped pair. The favorite wrong answer on this subtest is the neighbor: the water pump offered for the thermostat, the radiator for the water pump, the alternator for the battery. If two choices are a known pair, the question is about the boundary between them — that is what the next section is for.

4. For "what happens if" questions, follow the chain physically. One step is all any of these questions asks. A thermostat stuck shut means coolant never reaches the radiator, so the heat it carries has nowhere to go and the temperature gauge climbs steadily until the engine overheats. A fuse melts because too much current flowed, not because the voltage rose. The engine turns over slowly and the headlights go dim? Suspect a weak or discharged battery first. A badly underinflated tire flexes more as it rolls, so more of the engine's work is lost as heat, which hurts fuel mileage; an overinflated tire wears the middle of its tread and not its edges.

The computer, the dash, and what the car is telling you

Everything since the 1980s is run by a computer that decides how much fuel to inject, and it can only decide well if it is told what is happening — the sensors in the table above are how it is told. What the computer finds, it can report:

Under the dash, within reach of the driver's left knee on most cars, is the OBD-II port: the socket a technician plugs a scan tool into to read the fault codes the computer has stored. The codes are the reason diagnosis is not guesswork — a code names the circuit the computer is unhappy with, and the technician tests it.

The three emissions parts that are not the converter. The catalytic converter is the one everybody knows, but a modern car has three more parts doing the job upstream of it, and each handles a different kind of vapor or gas:

Question	What it handles	What it does
What does the PCV valve handle?	crankcase vapor (blow-by that gets past the piston rings)	routes it back into the intake so it is burned instead of vented
What does the EGR valve handle?	a small amount of exhaust gas	sends it back into the intake, which cools the burn and holds down the nitrogen oxides
What does the charcoal canister handle?	fuel vapor from the tank	holds it on charcoal until the engine can purge the canister and burn it

The distinctions to keep: crankcase / exhaust gas / fuel tank. The PCV valve is the only one dealing with the oil side, and the canister is the only one that stores anything — the question "which part holds fuel vapor from the tank instead of letting it escape" is the canister.

Why a failed oxygen sensor costs you an emissions test: a dead sensor leaves the computer with no feedback, so the mixture drifts rich or lean and the converter cannot clean what it was never designed to clean. The check-engine light comes on, the monitor for the converter never passes, and the car fails before anyone puts a probe in the tailpipe.

On the dash: which light means what

A warning light is a message with a severity attached, and the questions are about the severity as often as about the part.

Question	What it tells you	How urgent it is
What does the oil-pressure light (the oil can) tell you?	the oil is not circulating under pressure — it is not a level warning	the worst fault on the dash: metal is on metal within seconds, so shut the engine off
What does the coolant temperature gauge tell you?	how hot the coolant in the engine is	a needle climbing past its normal middle is the overheating chain running — stop before it runs on to destruction
What does the charging-system light (the battery shape) tell you?	the battery is not being kept up	an alternator or belt fault, not necessarily a dead cell — a faint glow at idle that goes when you rev the engine points the same way
What does the check-engine light tell you?	the computer has found a fault and stored a code	the mildest of the four: drive it to a scan tool, not to the shoulder of the road
What does the fuel gauge tell you?	how much fuel is left in the tank	the one reading here that is a level and not a fault — nothing has failed, the tank is just emptying

Checking the oil level afterwards is a separate job from the oil light: the light never told you the level was low, only that pressure was lost (a dead pump, a broken belt, a failed sensor).

If the question asks which light means stop now, it is the oil-pressure light. Temperature is a close second, but "stop the engine as soon as it is safe" is the oil light's sentence.

Service and safety: four ways a driveway job hurts people

None of this needs arithmetic, and all of it comes up — as "what does X do" and as "why must you not…". Each one is a physical chain, and the chain is the answer.

Question	What it is	The one fact to know
What are jack stands?	rigid stands that go under the frame and hold the car up once the jack has lifted it	nothing that is only held up by a jack may have a person under it — the stands go under the frame, on firm level ground, wheels chocked, with the jack left in place as a back-up
What is the radiator cap?	a pressure vessel, not a lid: the cooling system is sealed so the coolant can run above its normal boiling point	crack it while it is hot and that pressure is gone in an instant — the coolant boils and scalding coolant and steam come out of the filler neck, so wait for the engine to cool
What is battery gas?	a lead-acid battery being charged gives off hydrogen	it takes very little to set off — a cigarette, a spark from a dropped tool across the terminals, or the last jump lead — so ventilate and keep flames and sparks away

Jump-starting is the one sequence here, so it stays a sentence rather than a row. Positive to positive first (the dead car's positive, then the live car's), then the negative lead onto bare metal on the dead car's engine, away from the battery — never onto the dead battery's negative post. The reason is that the last connection made is the one that can spark, and a battery being charged gives off hydrogen. Remove the leads in the reverse order.

The confusables

Each line is a boundary this subtest is built on. Two choices from one row means the question is about that boundary.

Question	The boundary
How do alternator and battery differ?	makes electricity while the engine runs vs stores it for when the engine is off
How do radiator, water pump and thermostat differ?	dumps heat to the air vs circulates the coolant vs controls where the coolant may flow
How do master cylinder and caliper differ?	turns the pedal's force into hydraulic pressure vs presses the pads onto the disc
How do clutch and torque converter differ?	connects and disconnects a manual gearbox mechanically vs couples an automatic one through fluid
How do springs and shock absorbers differ?	hold the car up vs control the bounce so the tires keep contact with the road
How do dipstick and oil-pressure light differ?	shows how much oil is in the engine vs warns that the oil is not circulating under pressure
How do engine oil, coolant and brake fluid differ?	lubricates the engine vs carries heat away vs carries brake pressure
How do muffler and catalytic converter differ?	reduces noise vs turns harmful gas into less harmful gas
How do oxygen sensor and mass airflow sensor differ?	reads the exhaust on its way out vs measures the air on its way in
How do PCV valve, EGR valve and charcoal canister differ?	crankcase vapor vs a little exhaust gas sent back into the intake vs tank vapor held until the engine can burn it
How do oil-pressure light and coolant temperature gauge differ?	the oil has lost pressure, so stop the engine vs how hot the coolant is
How do oil-pressure light and check-engine light differ?	stop the engine now vs a fault was found and a code stored, drive to a scan tool
How do turbocharger and supercharger differ?	driven by the exhaust vs driven by a belt from the engine
How do overinflated and underinflated differ?	wears the middle of the tread vs flexes and hurts fuel mileage
What is the difference between the 215 and the 65 on a sidewall?	the tire's width in millimeters vs its height as a percentage of that width
How do differential and transmission differ?	lets the driven wheels turn at different speeds vs gives the engine several gear ratios
How do camshaft and crankshaft differ?	opens the valves at the right moment in the cycle vs collects the pistons' motion and hands it on as rotation
How do disc and drum differ?	a pad squeezes a disc from the outside vs a shoe presses out against a drum's inside
How do U-joint and CV joint differ?	a driveshaft's joint, where the axle angle changes as the car travels vs a driven wheel's joint that must also steer, at a steady speed

How to practice this

Work the free Auto Information practice set with the parts map in front of you. Every question on it is a function question, so read the part name, place it in a system, then predict the job before you look at the choices.
Say the four strokes out loud once a day, in order: intake, compression, power, exhaust. A stroke question is worth a point every time it appears, and that order is the answer.
Sketch the dash from memory and label each warning light with what it means and how urgent it is: oil pressure and temperature mean stop, charging means the battery is not being kept up, check engine means a code is stored. Half of the dash questions are about urgency rather than about the part.
Keep a swapped-pair list from the questions you get wrong. On this subtest the pairs are the questions: alternator against battery, clutch against torque converter, radiator against thermostat, oxygen sensor against mass airflow sensor, oil-pressure light against the dipstick.
Read the explanation on every question, including the ones you got right — the rationale for a wrong choice is usually the exact boundary you were fuzzy on.

The other half of this subtest's trade is the Shop Information guide — the tools, the fasteners and the materials. How to study for the ASVAB has the rest of the method, and what the pass adds.

Where to go next

Next topic — Shop Information
The exam itself — how the ASVAB is scored, how many questions, and what makes the ASVAB hard.

Auto Information carries 10 of the 135 scored questions on the computer test, where it is administered as one half of Auto & Shop Information and reported with the other half as a single score — and the AFQT score is not computed from it: that score comes from Arithmetic Reasoning, Mathematics Knowledge, Word Knowledge and Paragraph Comprehension.

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