- Oil viscosity (SAE)
- Oil's resistance to flow. In 5W-30, the W number is the cold-flow rating and the second number is the thickness at operating temperature.
- Multigrade oil (5W-30)
- Flows like a 5-weight when cold (the W) for easy starts and protects like a 30-weight at operating temperature.
- Oil-life monitor
- A system that estimates remaining oil life from engine data and signals when an oil change is due, replacing a fixed-mileage interval.
- Oil change interval
- Follow the owner's manual or oil-life monitor — commonly 5,000–10,000 miles on synthetic oil; oil degrades by both miles and time.
- Checking engine oil level
- Park on level ground, shut the engine off, wait a few minutes, then wipe, reinsert, and read the dipstick for an accurate level.
- Engine oil filter
- Traps contaminants from circulating oil; replaced with every oil change. A clogged filter can open a bypass and pass unfiltered oil.
- Synthetic vs conventional oil
- Synthetic oil resists breakdown better and usually allows longer change intervals; always use the grade and spec the manufacturer requires.
- Spark plug
- Delivers the spark that ignites the air-fuel mixture. Worn or fouled plugs cause rough idle, hesitation, and poor fuel economy.
- Iridium/long-life spark plug
- A long-life plug commonly replaced near 90,000–100,000 miles; always confirm the exact interval in the maintenance schedule.
- Fouled spark plug
- A plug coated with oil, fuel, or carbon that misfires; causes rough idle and poor power and should be diagnosed, not just replaced.
- Serpentine belt
- One multi-ribbed belt that drives accessories — alternator, A/C compressor, power-steering pump, and often the water pump.
- Belt tensioner
- A spring-loaded pulley that keeps the serpentine belt tight. A weak tensioner causes belt squeal and slipping.
- Belt squeal
- Usually a slipping serpentine belt or a weak/worn tensioner — not always a worn belt.
- EPDM belt wear
- Modern belts wear by losing rib material rather than cracking; check with a belt wear gauge.
- Coolant (antifreeze)
- A water-and-glycol mix that transfers heat and protects against freezing/corrosion. A 50/50 mix protects to about −34°C (−29°F).
- Checking coolant level
- Check cold at the reservoir; never open a hot radiator or pressure cap — escaping steam can cause burns.
- Thermostat
- A temperature valve that blocks flow to the radiator until the engine warms. Stuck closed = overheating; stuck open = runs too cool.
- Radiator cap
- Seals and pressurizes the cooling system, raising the coolant's boiling point so the engine can run hotter without boiling.
- Water pump
- The belt- or chain-driven pump that circulates coolant through the engine and radiator.
- Oil in the coolant
- Often a leaking head gasket letting oil and coolant passages mix; refer for deeper repair.
- Engine air filter
- Cleans intake air. A clogged air filter restricts airflow, richens the mixture, and can hurt economy and power.
- PCV system
- Positive Crankcase Ventilation — routes blow-by gases from the crankcase back to the intake to be burned, cutting pressure and emissions.
- Oxygen sensor
- Reads oxygen in the exhaust so the computer can keep the air-fuel ratio near the ideal 14.7:1 (stoichiometric).
- Stoichiometric ratio
- The ideal air-fuel ratio for gasoline — about 14.7 parts air to 1 part fuel by weight.
- Catalytic converter
- An exhaust device that converts CO, hydrocarbons, and NOx into harmless CO₂, water, nitrogen, and oxygen.
- Flashing check-engine light
- An active misfire severe enough to damage the catalytic converter — stop and diagnose the misfire.
- Steady check-engine light
- A stored emissions-related fault that is not an immediate threat; retrieve the code and diagnose.
- Compression test
- Measures cranking pressure per cylinder to judge sealing of rings, valves, and head gasket; readings should be within about 10%.
- Detonation (knock)
- Uncontrolled combustion after the spark, from low octane, too much advance, lean mixture, carbon, or overheating; the knock sensor retards timing.
- Knock sensor
- Detects detonation so the computer can retard ignition timing and protect the engine.
- Engine cooling fan
- Pulls air through the radiator at low speed/idle; an inoperative fan causes overheating in traffic but not at highway speed.
- Glow plugs
- Heat the combustion chamber on a diesel engine to aid cold starting; diesels have no spark ignition.
- Timing belt/chain
- Keeps the camshaft in time with the crankshaft; a failure on an interference engine can bend valves and stop compression.
- Mass airflow (MAF) sensor
- Measures intake air so the computer can meter the right fuel; a dirty MAF can cause misfires and poor drivability.
- Turbocharger boost
- Forced induction; less-than-expected boost is often a wastegate or control-system problem, checked first.
- Engine misfire
- A cylinder failing to fire — from ignition, fuel, or compression loss — causing rough idle, poor power, and a check-engine light.
- Vacuum leak
- Unmetered air entering the intake; leans the mixture and causes a rough or high idle.
- Fuel injector
- Sprays metered fuel into the intake or cylinder; a clogged or stuck injector causes a misfire or flooding.
- Engine overheating at high speed
- A partially obstructed radiator restricts flow and shows up most under high load and speed.
- Resetting maintenance reminder
- After an oil/maintenance service, reset the oil-life or maintenance reminder so the system tracks the next interval correctly.
- Automatic transmission fluid (ATF)
- The specified fluid an automatic uses for hydraulic control, lubrication, and cooling. Low or burnt ATF causes slipping and harsh shifts.
- Checking ATF level
- Check by the manufacturer's method — dipstick or a sealed check plug at a specified temperature; some units have no dipstick.
- Burnt ATF
- A burnt smell or very dark fluid signals overheating and the need for transmission service.
- Wrong ATF type
- Using a fluid that doesn't meet the manufacturer's spec can cause harsh or erratic shifting.
- Low ATF level
- Causes slipping, delayed engagement, and overheating; it is the first thing to check for transmission complaints.
- Torque converter
- A fluid coupling that transmits engine power to the transmission and multiplies torque; a slipping converter hurts economy and adds heat.
- Torque converter clutch (TCC)
- Locks the converter at cruise to eliminate slip and improve fuel economy.
- Planetary gearset
- The gear arrangement inside an automatic that provides the different gear ratios and reverse.
- Shift solenoid
- An electrically controlled valve that directs fluid to shift gears; a faulty solenoid can cause harsh or missed shifts.
- Pressure control solenoid
- Regulates line pressure; a defective one causes harsh or erratic shifting.
- Low line pressure
- Causes early, soft shifts because clutches and bands cannot engage firmly.
- Transmission filter
- Strains the ATF; a restricted filter causes delayed engagement and a whining pump.
- Valve body
- The hydraulic control center of an automatic; sticking valves (often when cold) cause delayed engagement.
- Transmission control module (TCM)
- The computer that controls shifting; scanning it for codes helps diagnose electronic shift faults.
- Slipping in overdrive
- Often a loose overdrive band or worn clutch pack — the transmission slips momentarily on the OD shift.
- Delayed engagement when cold
- Often sticking valves in the valve body that free up as the transmission warms.
- No movement in any gear
- Check ATF level first — a low level is the most common and simplest cause.
- Reverse clutch pack
- Engages reverse gear; a failure can leave reverse inoperative while forward gears work.
- Forward clutch
- Engages forward gears; if not seated, the transmission has no forward movement but reverse may work.
- Transmission cooler
- Routes ATF through a cooler (often in the radiator) to control fluid temperature.
- ATF service
- Drain-and-fill or full exchange with the specified fluid; follow the maintenance schedule's interval.
- Limp-in mode
- A protective default to one gear when the TCM detects a serious fault, to let the vehicle reach a shop.
- Clutch
- Connects and disconnects engine power to a manual transmission; lets the driver change gears and start from a stop.
- Clutch disc
- The friction plate clamped between the flywheel and pressure plate; wear causes slipping and a high engagement point.
- Pressure plate
- Clamps the clutch disc to the flywheel; releasing it disengages the clutch.
- Throw-out (release) bearing
- Pushes on the pressure plate fingers to disengage the clutch; a worn one whines when the pedal is pressed.
- Pilot bearing
- Supports the transmission input shaft in the crankshaft; a worn pilot bearing whines with the clutch engaged.
- Air in hydraulic clutch
- Prevents the clutch from fully releasing, making it hard to shift into all gears; bleed the system.
- Synchronizer
- Matches gear speeds so a manual transmission shifts smoothly; worn synchros cause grinding and popping out of gear.
- Grinding into reverse
- Often a faulty reverse idler gear — reverse usually has no synchronizer.
- Pops out of gear
- Worn or damaged synchronizers (or worn detents) let a manual transmission jump out of gear.
- Constant-velocity (CV) joint
- A flexible drive-axle joint that transmits power while allowing steering and suspension movement.
- Torn CV boot
- Lets grease out and dirt/water in, destroying the CV joint; inspect boots at every service.
- CV joint clicking
- A rhythmic click during tight turns is the classic worn outer CV joint symptom.
- Driveshaft
- Transmits power from the transmission to the rear axle on RWD vehicles; a bent shaft causes a speed-related vibration.
- U-joint
- A universal joint that lets the driveshaft flex; a worn U-joint causes a clunk on acceleration and a vibration.
- Differential
- A gearset that lets the drive wheels turn at different speeds, needed because the outer wheel travels farther in a turn.
- Ring and pinion
- The differential gears that turn drive from the driveshaft 90° to the axles and provide final gear reduction.
- Metal in differential fluid
- Metal flakes usually indicate failing differential bearings.
- Gear oil
- The lubricant for manual transmissions, transaxles, and differentials; low level causes noise and wear.
- Transfer case
- Splits drive to front and rear axles on 4WD vehicles; a defective shift motor can fail to engage 4WD.
- Wheel bearing noise
- A constant hum that changes with vehicle speed (not steering angle), unlike a CV joint click.
- Clutch slipping
- Engine RPM rises without matching acceleration; a worn clutch disc or oil-contaminated clutch is the cause.
- Axle seal
- Keeps gear oil in the differential/axle; a leaking seal causes fluid loss and possible contamination of nearby parts.
- Camber
- The inward/outward tilt of a wheel viewed from the front; excessive camber wears one edge of the tire.
- Caster
- The forward/rearward tilt of the steering axis viewed from the side; controls straight-line stability and steering return, not tire wear.
- Toe
- Whether the tires point in or out; incorrect toe is the most common cause of feathered tire wear.
- Toe-in / toe-out
- Toe-in: fronts of the tires point toward each other; toe-out: they point away. Wrong toe scrubs and feathers the tread.
- Positive camber
- The tops of the wheels tilt outward; excessive positive camber wears the outer edges of the tires.
- Negative camber
- The tops of the wheels tilt inward; excessive negative camber wears the inner edges of the tires.
- Wheel alignment
- Setting camber, caster, and toe to spec for even tire wear and stable handling; required after replacing related parts.
- Tire rotation
- Moving tires between positions for even wear and longer life; pattern depends on drive type and tire directionality.
- Tire inflation source
- Inflate to the door-jamb placard pressure, NOT the maximum stamped on the sidewall.
- Checking tire pressure cold
- Check when tires are cold — driving heats the air and raises the reading.
- Under-inflation wear
- Wears both edges/shoulders of the tread and runs the tire hot.
- Over-inflation wear
- Wears the center of the tread.
- Feathered tire wear
- A saw-tooth edge pattern caused by incorrect toe.
- Cupping/scalloping
- Uneven dips in the tread from worn shocks/struts or other suspension wear.
- TPMS
- Tire Pressure Monitoring System — warns of low pressure; many vehicles need a relearn after tire service.
- Direct vs indirect TPMS
- Direct uses a sensor in each wheel; indirect infers pressure from wheel-speed differences.
- TPMS light solid vs flashing
- A solid light means low tire pressure; a flashing light usually means a sensor or system fault.
- Shock absorber
- Damps spring oscillation; worn shocks cause excessive bounce, nose-dive, and tire cupping.
- Strut / MacPherson strut
- A combined shock and suspension upright that also serves as a steering pivot, common on front suspensions.
- Coil spring
- Supports vehicle weight and absorbs road shock; a broken/sagging spring lets a corner sit low.
- Leaf spring
- A stacked-blade spring (often rear); worn leaves let the rear sag.
- Ball joint
- A pivoting connection between control arm and knuckle; a worn one causes a clunk over bumps and uneven tire wear.
- Control arm bushing
- Cushions the control arm pivot; worn bushings cause clunks and alignment changes.
- Sway bar (stabilizer) link
- Connects the sway bar to the suspension; worn links rattle/clunk over bumps.
- Tie rod end
- Connects the steering linkage to the knuckle; wear causes excessive steering play and a bent tie rod alters toe.
- Rack and pinion
- A common steering gear that converts steering-wheel rotation to side-to-side motion; worn mounts cause a clunk.
- Idler arm / pitman arm
- Steering-linkage parts in a parallelogram system; wear causes play and wander.
- Power steering pump
- Provides hydraulic assist; a weak pump or low fluid causes hard steering, worst at low speed.
- Electric power steering
- Uses an electric motor instead of a hydraulic pump; no power-steering fluid to service.
- Steering wheel shake at highway speed
- Usually unbalanced wheels causing vibration felt through the wheel.
- Off-center steering wheel
- A steering wheel not centered when driving straight, often from a bent tie rod or an uncorrected alignment.
- Excessive steering play
- Often worn tie-rod ends or a worn steering gear; inspect linkage first.
- Strut mount bearing
- Lets the strut rotate with steering; a faulty bearing squeaks/binds during turns.
- Wheel bearing
- Supports the wheel and lets it spin; a worn bearing growls and changes with vehicle speed.
- Clunk over bumps
- Often worn ball joints, control-arm bushings, sway-bar links, or strut mounts.
- Wheel balancing
- Adding weights so a wheel/tire spins without vibration; imbalance causes shake at speed.
- Ride height
- The vehicle's resting height; a low corner indicates a weak or broken spring.
- Steering damper
- Dampens steering oscillation (common on trucks); not a tire-wear item.
- Master cylinder
- The pedal-operated hydraulic pump that sends pressurized fluid to all four wheels; an internal leak lets the pedal slowly sink.
- Brake fluid (DOT)
- A hygroscopic hydraulic fluid (DOT 3/4/5.1) that absorbs moisture, lowering its boiling point; keep capped and change on schedule.
- Hygroscopic brake fluid
- Brake fluid absorbs water over time, which lowers the boiling point and can cause a spongy pedal under heavy use.
- Brake bleeding
- Removing air from the hydraulic system to cure a soft/spongy pedal.
- Soft/spongy brake pedal
- Usually air in the hydraulic system; bleed the brakes.
- Sinking brake pedal
- A pedal that slowly drops under steady pressure points to a failing master cylinder (internal leak).
- Disc brake
- Pads clamp a spinning rotor to stop the wheel; common on front axles and many rears.
- Brake pad
- The friction material that clamps the rotor; replace as a set per axle when worn.
- Brake pad wear indicator
- A metal tab that contacts the rotor when pads are thin, making a high-pitched squeal that warns of replacement.
- Brake rotor (disc)
- The disc the pads clamp; a warped or thickness-varied rotor causes a pedal pulsation.
- Rotor minimum thickness
- The discard spec stamped on the rotor; a rotor below it must be replaced, not machined.
- Brake pedal pulsation
- A rhythmic pulse felt during braking from a warped or uneven-thickness rotor.
- Brake caliper
- Houses the pistons that press the pads against the rotor; a sticking caliper drags and can cause a pull.
- Sticking caliper
- Causes a brake drag, uneven pad/dust, and a pull — usually the vehicle pulls toward the OPPOSITE side that is sticking.
- Brake pull
- A pull to one side under braking, usually a sticking caliper on the opposite side or a hydraulic restriction.
- Drum brake
- A rear-brake design where shoes press outward against a rotating drum.
- Brake shoes
- The friction parts in a drum brake that press against the drum.
- Wheel cylinder
- The hydraulic actuator in a drum brake that pushes the shoes out; a leak contaminates the linings.
- Self-adjuster (drum)
- Keeps drum-brake shoe-to-drum clearance correct as the linings wear.
- Parking brake
- A mechanical brake (cable or electric) that holds the vehicle; it must be adjusted for proper holding.
- Brake booster
- A vacuum- or hydraulically-assisted unit that multiplies pedal force; a failure makes the pedal very hard.
- Hard brake pedal
- Excessive pedal effort with adequate stopping usually means a failed booster or lost vacuum.
- Vacuum booster check valve
- Holds vacuum in the booster; a leak gives a soft pedal at start-up that firms once the engine runs.
- Antilock braking system (ABS)
- Pulses brake pressure to keep wheels from locking during hard braking, preserving steering control.
- ABS wheel speed sensor
- Reports each wheel's speed to the ABS module; a faulty sensor can cause improper ABS operation.
- ABS warning light
- Lights for an ABS fault; first scan for ABS trouble codes before replacing parts.
- Brake fade
- Loss of braking from overheated pads/fluid; boiling fluid (water-laden) makes it worse.
- Proportioning valve
- Balances front/rear brake pressure to prevent rear lock-up under hard braking.
- Residual pressure
- A small held pressure that, if excessive, can cause brakes to drag and not fully release.
- Brake dust
- Pad/rotor wear dust; much less on one wheel can indicate a seized caliper there.
- Squeal that stops when braking
- A wear-indicator squeal at light braking that quiets when the pedal is firmly applied.
- Measuring rotor runout
- Checking lateral wobble with a dial indicator; excess runout causes pulsation.
- Battery (12V)
- Stores electrical energy to crank the engine and run accessories when the engine is off.
- Battery terminal corrosion
- Adds resistance and can cause slow cranking and starting problems; clean and secure terminals as service.
- Battery hold-down
- Secures the battery to prevent vibration damage and shorts; tighten as routine service.
- Battery load test
- Applies a load to check the battery's ability to deliver cranking current; a conductance tester is also used.
- Alternator
- The belt-driven generator that charges the battery and powers loads, typically 13.5–14.5 volts running.
- Charging-system check
- Confirms the alternator keeps the battery charged at about 13.5–14.5 V at idle.
- Battery warning light
- On with the engine running, it indicates a charging-system problem (not just a low battery).
- Headlights dim at idle
- Dimming as RPM drops at idle suggests a failing alternator that cannot keep up at low speed.
- Corroded alternator B+ terminal
- Adds resistance and can leave the battery undercharged.
- Starter motor
- The electric motor that cranks the engine; draws high current through the starter circuit and solenoid.
- Starter solenoid
- An electromagnetic switch that engages the starter and connects high current to it.
- Starter relay
- A control relay in the starting circuit; a defective relay can prevent cranking.
- Slow cranking (good battery)
- Usually high resistance in the starter circuit — corroded/loose cables or grounds.
- Voltage-drop test
- Measures voltage lost across a connection under load; above about 0.1 V on a ground means unwanted resistance.
- Ground connection
- The return path of a circuit; a poor ground causes dim lights, slow crank, and odd electrical faults.
- Fuse
- A sacrificial link that opens to protect a circuit from overcurrent; check the fuse first for a dead circuit.
- Relay
- An electrically operated switch that lets a small current control a larger one (fans, fuel pump, starter).
- OBD-II / DLC
- The on-board diagnostics connector for scan tools; check its fuse if the scan tool won't communicate.
- Test light vs DMM
- A test light shows presence of voltage; a digital multimeter measures exact volts, ohms, and amps.
- Short circuit
- An unintended low-resistance path that blows fuses or drains the battery.
- Open circuit
- A break in the circuit that stops current flow; the load won't operate.
- Parasitic draw
- Current drawn with the key off that drains the battery overnight; measured in milliamps.
- Cooling fan relay (electrical)
- Controls the electric cooling fan; a shorted relay can run the fan constantly.
- Headlight/bulb replacement
- A common light repair; match the correct bulb type and avoid touching the glass on halogen bulbs.
- Refrigerant
- The fluid (R-1234yf or R-134a) an A/C system circulates to move heat from the cabin to outside; it must be recovered, never vented.
- R-1234yf vs R-134a
- R-1234yf is the newer low-GWP refrigerant in late-model vehicles; R-134a is older. They must not be mixed.
- A/C compressor
- Pumps and pressurizes refrigerant; its clutch engages it. A seized pulley bearing causes noise and belt slip.
- A/C condenser
- Sheds heat from high-pressure refrigerant (like a small radiator) at the front of the vehicle.
- A/C evaporator
- Absorbs cabin heat as refrigerant boils; condensation on it drains outside and can grow mold.
- Expansion valve / orifice tube
- Meters refrigerant into the evaporator, dropping its pressure so it can absorb heat.
- Refrigerant recovery
- Refrigerant must be captured with a recovery/recycling machine, never released to the atmosphere.
- A/C system pressures
- Checked with manifold gauges; abnormal high/low readings point to charge level or component faults.
- Low refrigerant charge
- Causes poor cooling; bubbles in a sight glass at idle can indicate a low charge.
- Musty A/C odor
- Mold or mildew growth on the evaporator and case; worsened by a clogged cabin filter or blocked drain.
- Cabin air filter
- Cleans air entering the cabin; when clogged it reduces airflow on all settings and can cause odors.
- Weak airflow on all settings
- A classic clogged cabin air filter symptom; replace the filter.
- Evaporator drain
- Carries condensation outside; a blocked drain leaves water in the case and can wet the carpet.
- Heater core
- A small radiator that uses hot engine coolant to warm cabin air.
- Leaking heater core
- Causes a sweet antifreeze smell, foggy windows, and sometimes wet carpet.
- Heater control (blend) valve/door
- Controls how much hot coolant/air reaches the cabin; a fault causes poor heat or temperature control.
- Blend door actuator
- A motor that positions the blend door for temperature/mode; a failure causes wrong temperature or mode.
- Blower motor
- Moves air through the HVAC system; a clogged cabin filter strains it and weakens airflow.
- HVAC mode selection
- Directs airflow to panel, floor, or defrost vents; a vacuum leak can default the system to defrost.
- High-pressure cut-off switch
- Shuts off the compressor on overpressure; a trip often points to poor condenser cooling (check the fans).
- A/C short cycling
- The compressor cycling on/off too often, commonly from a low refrigerant charge.
- Defrost mode
- Directs warm, dehumidified air to the windshield; the A/C often runs in defrost to remove moisture.