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What fluids are in a car?

What fluids are in a car?

There are several fluids running around your engine, and it's wise to know how to check their levels – even if you don't plan on changing them yourself.

Most of the fluids in your engine compartment are critical for the car’s operation. Among the most important are the engine oil and coolant, this is why these should be checked weekly.

A low engine oil level will cause the engine to wear more quickly, and if it drops to a dangerous level the engine could seize. It’s a similar story with the coolant.

Brake, clutch and power steering fluid are still important fluids but don’t need to be checked as often.

However, a drop in level from month to month indicates a leak in the system, and it’s important to fix this as soon as possible, rather than to keep topping up the fluid.

Weekly fluid checks include: engine oil, engine coolant and windshield washer fluid (if possible).

Monthly fluid checks include: battery electrolyte (if the battery isn’t sealed), brake and clutch fluid.


Oil lubricates the moving parts of the engine, reducing wear. It also assists in cooling the engine. Think of it as the engine's lifeblood.

Look after it, keep it at the correct level and it will prolong the life of your engine.

Engine oils come in three main varieties - synthetic, semi-synthetic, and mineral. Mineral oil offers the least protection, don’t last as long, and don't cope as well with extreme temperatures.

As a result its use is generally confined to older vehicles. Synthetic and semi-synthetic oils are specially formulated blends, which come in a variety of different viscosities and temperature ratings, depending on use, and application.

Discover how to change the oil on your car


This liquid flows around the engine, radiator and heater matrix. It prevents the engine from overheating and allows the heaters to work. Coolant also contains anti-freeze.

This, as the name suggests, prevents the coolant from freezing. If it did freeze it could well damage the engine and/or radiator.

There are different types of coolant depending on application and you may see them referred to as IAT (Inorganic Acid Technology), OAT (Organic Acid Technology), and HOAT (Hybrid Organic Acid Technology).

It's always best to put the same type in your car if you're topping it up, as mixing them can shorten their service life. 

Discover how to change the coolant in your car

Washer fluid

The stuff that comes out of the windscreen washers is stored in a reservoir that's commonly found in the engine bay (and occasionally in the rear of the car too).

It's formulated to help remove road grime, not 'foam up' too much and resist freezing in winter.

It's always best to use the correct fluid and not washing up liquid or other household soap.

Steering fluid

Many cars are fitted with power steering to make it easier for the driver to turn the steering wheel. Some heavy cars with wide, low-profile tyres would be almost impossible to steer at low speeds without power steering.

Most power steering systems use hydraulic pressure to increase the effort applied to the steering wheel by the driver, with an engine-driven pump supplying the hydraulic pressure. Some cars use an electric power steering system.

Your car’s power steering fluid reservoir is located in the engine bay. Look for the MAX-MIN positions on the side of the translucent body.

Some power steering fluid reservoirs have a level dipstick attached to the filler cap, while others have a transparent reservoir with markings on the outside, similar to your brake fluid reservoir.

Sometimes there may also be HOT or COLD level markings for use depending on whether the engine is hot or cold

Brake fluid

Most cars use conventional hydraulic fluid in the brake and clutch systems, but it is worth checking in your car handbook to see which type of fluid is recommended.

Usually the specification will be for ‘DOT 3’, ‘DOT 4’ or ‘DOT 5’ hydraulic fluid – always make sure that you use the correct type.

The ‘DOT’ specification will usually be marked clearly on the fluid packaging.

Brake fluid should always be stored in a closed, airtight container, as it absorbs water from the atmosphere, which will cause the fluid to deteriorate. Always use clean fluid from a sealed container for topping up purposes.

How it works is, your car’s brake pedal operates a piston inside a master cylinder full of fluid. When the pedal is pressed, the piston moves, causing fluid to move from the master cylinder along a narrow pipe.

The movement of the fluid moves a second piston at the other end of the pipe, which operates the brake.

If there’s a leak in the hydraulic system the fluid can escape, so the brakes won’t work properly.

Modern braking systems are efficient and reliable, but they must be maintained properly to ensure safety. The fluid in the braking system deteriorates with age, and it must be renewed at the manufacturer’s recommended intervals – usually every two years.

Discover how to change your car's brake fluid

Clutch fluid

Not all cars have a clutch fluid reservoir because not all cars have a hydraulic clutch. And even some of those that do have a sealed hydraulic system where the level can’t be checked.

It’s worth checking your car’s handbook for details on what type of clutch is fitted. If you do have a clutch fluid reservoir, it’s worth checking its levels each week.

Automatic Transmission fluid

As a rough guide, if your car has an automatic transmission the fluid level should be checked around every 12 months or 6,000 miles (10,000km).

If the automatic transmission fluid level gets low the transmission may not work properly – low fluid level is a common source of problems with automatic transmissions.

If the level drops too far it could even damage the transmission. There are several different types of automatic transmission fluid and it’s essential that you use the right one, so you’ll need to check your car’s handbook, or with an authorised dealer. If frequent topping-up is needed have the cause found and fixed without delay.

The following advice is only a guide – you need to check your car’s handbook to see exactly how to check the automatic transmission fluid level, but the technique is similar for most cars, using a level dipstick which fits inside a tube attached to the transmission.

The level is usually checked with the transmission warm, after a short drive.