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Maintenance and tyres , Know your car , Safety and breakdowns · Updated 2026-10-18

Coolant and other fluids: a simple check

A car depends on several fluids, and most of them can be checked in a few minutes without any tools. A short look under the bonnet once a month, and before long trips, catches many problems before they become breakdowns. This guide covers what each fluid does, how to check it and when to call a workshop.

Safety first

• Park on level ground and let a hot engine cool before opening anything.
• Never open the coolant cap on a hot engine: the system is under pressure and can spray scalding liquid.
• Keep hands, hair and loose clothing away from belts and the fan, which can start on its own.
• Many fluids are poisonous. Keep them away from children and pets, and wipe up spills.

Coolant

Coolant, often called antifreeze, carries heat from the engine to the radiator. It also protects against freezing and corrosion. The expansion tank is usually translucent with minimum and maximum marks. Check the level when the engine is cold; it should be between the marks.

The liquid should look clean and have a normal colour for the product. Rusty, oily or muddy coolant suggests a problem. A level that keeps falling means a leak or, in worse cases, an internal engine fault. Do not just keep topping up: have it checked. If you must top up, use the type the manual specifies, because coolants of different types should not be mixed carelessly. Plain water is only an emergency measure and weakens the protection.

The temperature gauge or a warning light shows overheating. If it climbs into the red, stop safely and switch off the engine. See the summer heat checklist for more on hot-weather problems.

Engine oil

The oil level is checked with the dipstick when the engine has been off for a few minutes. The full steps are in engine oil explained.

Brake fluid

The reservoir sits near the back of the engine bay with minimum and maximum marks. You can look at the level, but do not top it up yourself without advice. A falling level normally means worn pads or a leak, and the brakes are too important to guess about. See brake basics and wear signs.

Power steering fluid

Many older cars have a hydraulic power steering with its own reservoir. If the steering becomes heavy or whines when you turn, check the level. Many newer cars use electric steering and have no fluid at all. The manual tells you which one you have.

Windscreen washer fluid

This is the easiest and most useful top-up. In cold weather use a mix that will not freeze, as plain water can freeze in the pipes and crack the reservoir. Summer mixes help remove insects. Do not put coolant or any other fluid in the washer tank.

Gearbox and other oils

Gearbox, differential and automatic transmission oils are normally not checked by owners. They are changed at the intervals in the service schedule, and some are described by makers as "lifetime", which in practice is a long but finite time. Leaks or changes in gear shifting should be examined by a workshop.

Reading leaks under the car

A damp patch on the ground tells a story. As a rough guide:

• Clear water under the front on a hot day is usually condensation from the air conditioning and is normal.
• Green, pink, orange or blue liquid with a sweetish smell is likely coolant.
• Dark brown or black, slippery liquid is likely engine oil.
• Clear or yellowish, oily liquid near a wheel may be brake fluid. Treat this as urgent.
• Reddish liquid may be transmission or power steering fluid.

Put a piece of cardboard under the car overnight to see where the drips fall, and show it to the workshop.

A simple monthly routine

1. Park on level ground with the engine cold.
2. Check coolant, oil, brake fluid and washer fluid levels.
3. Look for wet spots on the engine and on the ground.
4. Check the tyres and lights while you are there, using the tyre pressure guide.
5. Write anything unusual in your log in My Garage.

When buying a used car

Fluid levels and condition are an easy part of an inspection and say a lot about how the car was treated. Add them to your list from how to inspect a used car.

What to remember

• Check fluids on level ground and, for coolant, only when the engine is cold.
• Never open a hot coolant system.
• A level that keeps falling means a leak: find the cause instead of topping up forever.
• Do not guess with brake fluid; see a workshop.
• Use the fluid types in the manual and never mix up the tanks.

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