Cause library
What actually fails, and how to prove it
Symptom pages start with what your car is doing. These start one level deeper — with the component itself. What it does, how it fails, the specific test that confirms it, what the repair should cost, and which quotes deserve a second opinion.
Every cause here links back to every symptom it can produce, because the same failed part shows up in several different complaints. If your symptom has more than one possible cause, this is where you find out how a shop is supposed to tell them apart.
34 causes, each with the test that proves it
Weak or dying battery
Look at it soon · 5 symptoms
The single most common cause of no-starts, and the one most often replaced when something else is at fault.
Read itCharging system and alternator failure
Urgent · 5 symptoms
When the alternator stops keeping up, the car runs on stored battery power until it runs out — usually mid-drive.
Read itParasitic draw
Look at it soon · 2 symptoms
Something on the car stays awake after you lock it and quietly empties the battery while you sleep.
Read itIgnition or misfire fault
Look at it soon · 3 symptoms
One cylinder stops contributing, so the engine shakes, stumbles and usually sets a code naming which one.
Read itVacuum or intake leak
Fix when convenient · 2 symptoms
Air sneaks into the engine after the sensor that measures it, so the computer's fuel math is wrong — worst at idle.
Read itCooling system failure
Urgent · 6 symptoms
A thermostat, water pump, radiator or fan that can no longer shed heat as fast as the engine makes it.
Read itFluid leak onto a hot surface
Look at it soon · 3 symptoms
The most common source of a burning smell, and the reason a small leak deserves attention before it becomes smoke.
Read itWorn brake pads or rotors
Look at it soon · 4 symptoms
Normal wear that squeals as a designed warning and grinds once the friction material is gone.
Read itSticking caliper or dragging brake
Look at it soon · 6 symptoms
One brake never fully releases, which heats a wheel, pulls the car, kills fuel economy and smells hot after a drive.
Read itWheel balance and tire imbalance
Fix when convenient · 2 symptoms
A wheel that is heavier on one side shakes in a narrow speed band, usually somewhere between 55 and 70 mph.
Read itTire damage or belt separation
Urgent · 4 symptoms
A belt shifting inside the tire creates a vibration that worsens with every mile and cannot be balanced out.
Read itAlignment and tire pressure
Fix when convenient · 5 symptoms
Uneven pressure or a wheel pointing slightly off pulls the car and wears one edge of a tire early.
Read itWorn suspension and steering components
Look at it soon · 6 symptoms
Bushings, ball joints, tie rods and struts wear out and let a wheel move where it should not.
Read itWheel bearing or CV joint
Look at it soon · 3 symptoms
A hum or grind that changes with speed or steering, and gets louder every month you leave it.
Read itLow or degraded transmission fluid
Look at it soon · 4 symptoms
The cheapest cause of slipping — and the one that stops being cheap the longer you drive on it.
Read itInternal transmission wear
Look at it soon · 3 symptoms
Clutch packs or a torque converter that no longer hold under load — the expensive end of the diagnosis.
Read itRefrigerant leak
Fix when convenient · 2 symptoms
A/C is a sealed system. If it is low, it leaked — and a recharge without a leak search is a countdown, not a fix.
Read itCondenser airflow or cooling fan
Look at it soon · 3 symptoms
A blocked condenser or a fan that is not pulling air is why some cars only cool while moving.
Read itBrake fluid, air or master cylinder
Urgent · 3 symptoms
The hydraulic side of the brakes: old fluid that boils on grades, air in the lines, or a master cylinder bypassing internally.
Read itHeat-sensitive sensor failure
Urgent · 3 symptoms
A crankshaft or camshaft position sensor that works cold and drops out once under-hood temperature climbs.
Read itFuel delivery and fuel pump faults
Look at it soon · 4 symptoms
A weak pump, clogged filter or failing injector that cannot keep up under load — and often leaks vapor as it ages.
Read itEvaporative emissions leak
Fix when convenient · 2 symptoms
A hardened gas cap seal, purge valve or heat-cracked vapor hose letting fuel vapor escape where you can smell it.
Read itTPMS sensor or slow leak
Look at it soon · 2 symptoms
A dead sensor battery, a nail in the tread, or desert temperature swings pushing a tire below the warning threshold.
Read itShift control, solenoid or adaptive relearn
Look at it soon · 3 symptoms
Valve body and solenoid faults, or a transmission that lost its learned shift pressure after a battery change.
Read itLow coolant or stuck thermostat
Look at it soon · 3 symptoms
The heater runs on engine coolant, so it goes cold first when the level drops or the engine never fully warms.
Read itRotor thickness variation
Look at it soon · 3 symptoms
Heat leaves hard spots and uneven thickness across a rotor, so the pad pushes back once per wheel revolution.
Read itSteering linkage play
Look at it soon · 3 symptoms
Small amounts of wear in several joints add up to a wheel that no longer goes exactly where you pointed it.
Read itCorroded ground or connector
Look at it soon · 3 symptoms
Every circuit ends at a bare metal ground. When that bolt corrodes, resistance climbs and strange things happen at once.
Read itWindow regulator, switch or door wiring
Fix when convenient · 1 symptom
A frayed regulator cable, a worn master switch, or wires broken inside the flexible boot at the door hinge.
Read itSerpentine belt or tensioner
Look at it soon · 3 symptoms
A glazed belt or a weak tensioner lets the belt slip, which can cost you charging, cooling and steering assist all at once.
Read itOil burning past seals or rings
Look at it soon · 3 symptoms
Hardened valve stem seals or worn rings let oil into the combustion chamber — blue smoke and a dropping dipstick.
Read itHead gasket or coolant in a cylinder
Urgent · 3 symptoms
The diagnosis worth confirming with a real test before you approve it — and the one people are told most often.
Read itRich running fuel mixture
Look at it soon · 3 symptoms
The engine burns more fuel than it can use — black smoke, poor economy, and a catalytic converter on borrowed time.
Read itBlend door or HVAC actuator
Fix when convenient · 2 symptoms
A small electric actuator behind the dash stops moving, so the air never changes temperature no matter what you select.
Read itStart from the symptom instead
If you would rather begin with what your car is doing, the symptom library walks the whole pattern and points back to the causes here.