Under the hood — fluids in your car explained
Driving-school guide to what is under the hood — four operating fluids (engine oil, coolant, brake fluid, washer fluid) plus the battery and alternator that power the entire car electrical system.
Under the hood — what fluids are in a car and why they matter
Every modern car runs on a set of operating fluids that lubricate, cool, transfer force and keep visibility clear. Driving schools teach you to recognise all of them under the bonnet, check their level and top them up — because a missing or contaminated fluid can cause an instant breakdown or a failed technical inspection (STK).
Below you will find each fluid explained: what it does, where the reservoir is, how to read the dipstick or sight glass, and how often to check it.
Operating fluids in a car — full overview
Four fluids you will find under the bonnet of every passenger car.
Why driving schools teach this
The driving examination (závěrečná zkouška) includes a practical knowledge check at the start of the driving exam — the examiner asks you to identify components under the bonnet and explain what to check before driving. A confident answer about fluids almost always passes; hesitation is a frequent reason for a re-take.
Engine oil
- Lubricates the engine — reduces friction and wear
- Checked via dipstick — between MIN and MAX marks
- Top up via yellow oil filler cap
- Full change every 15 000 km or once a year
Coolant
- Keeps engine at optimum 90 °C
- Pink/green/blue colour in a transparent expansion tank
- Check only with engine COLD — risk of scalding
- Contains antifreeze — never use plain water
Brake fluid
- Transfers force from pedal to brake calipers
- Small reservoir near windscreen, DOT 4 standard
- Hygroscopic — absorbs water; change every 2 years
- Low level = warning light — STOP driving immediately
Washer fluid
- Cleans windscreen — required for road safety
- Filler with blue cap with windscreen + water symbol
- Summer = bug remover; winter = antifreeze to −20 °C
- Empty reservoir = fail at the STK technical inspection
Battery and alternator — electrical system under the hood
Two components that power and recharge everything from headlights to the radio.
How they work together — in one sentence
The battery stores electricity and supplies it to the starter motor and all electronics when the engine is off. The moment the engine starts, the alternator (a small generator belt-driven by the engine) takes over — it powers everything and refills the battery so it is ready for the next start. Without a working alternator the car runs only until the battery drains (typically 30–60 minutes).
Battery (accumulator)
- 12V lead-acid box, typically on the left of the engine bay
- Powers the starter motor — the very first job at every start
- Two terminals: red (+) and black (−) — never short-circuit
- Lifespan 4–6 years; replace at first signs of slow cranking
Alternator
- Small generator belt-driven by the engine
- After start: powers everything AND recharges the battery
- Output ~14 V (higher than battery so current flows IN)
- Red battery warning light = alternator not charging — go to service
How to check fluids in a car
Five-minute routine you can do at home — before a longer trip or once a month.
Before you start
Park the car on a level surface, switch off the engine and let it cool for at least 10 minutes. Open the bonnet — the release lever is under the dashboard on the driver's side, then a second latch under the bonnet itself. Use a clean cloth and a pair of disposable gloves; some fluids stain and some are aggressive to skin.
Step-by-step
- Engine oil — pull the dipstick, wipe clean, reinsert fully, pull again. Oil mark must be between MIN and MAX.
- Coolant — read the level on the transparent expansion tank against MIN/MAX marks. Never open the cap on a hot engine.
- Brake fluid — small reservoir, read level through the wall against MIN/MAX. Low level = STOP, do not drive.
- Washer fluid — open the blue cap and look in. Top up to the neck.
Quick visual check — battery and alternator belt
While the bonnet is open, spend ten seconds on the electrical side too: check the battery terminals are clean (no greenish-white powder around the clamps) and that the alternator belt looks intact — no cracks or shiny strips. A broken belt = no charging within minutes.
Frequent questions about car fluids
What fluids are in a car?
A standard passenger car (category B) contains four operating fluids you check yourself: engine oil, coolant, brake fluid and windscreen washer fluid. Plus two electrical components under the hood — battery and alternator.
How often should I check the fluids?
Quick monthly check of all four reservoirs; full check before a long trip (over 300 km); seasonal swap of washer fluid in October and April. Engine oil is worth a glance at every refuel.
Can I mix different coolant colours?
No — coolants of different specifications (G11, G12, G13) can react and form deposits that clog the cooling system. If you do not know the exact spec, top up with distilled water for the trip home and have a service flush + refill.
What happens if I drive with low brake fluid?
The brake warning light comes on and pedal travel grows longer. Continued driving risks total brake failure. Stop immediately, top up with the prescribed DOT 4, and go straight to a service to find the leak.
How do the battery and alternator work together?
The battery supplies power when the engine is off (starter, alarm, dashboard). Once the engine runs, the alternator — a belt-driven generator — takes over and powers everything and recharges the battery (at ~14 V, higher than the battery so current flows in). A red battery warning light while driving means the alternator stopped charging — head to a service.