Checking the oil and other fluids under the hood

Check each fluid on level ground the way your owner's manual says, and never open a radiator or coolant cap while the engine is hot.

Checking the fluids under the hood. Read the oil dipstick once the engine has been off for at least 15 minutes, and, with the engine cool, read coolant and brake fluid through the side of their reservoirs, between MIN and MAX. Never open the radiator or coolant reservoir cap on a hot engine; the cooling fan can start even with the engine off. — **Fig. 9.16** Read the oil dipstick once the engine has been off for at least 15 minutes, and, with the engine cool, read coolant and brake fluid through the side of their reservoirs, between MIN and MAX. Never open the radiator or coolant reservoir cap on a hot engine; the cooling fan can start even with the engine off. Source: Ford Canada; Kia 2024 Sportage, Honda 2007 Accord and Mazda3 owner's manuals.

Area: HandsFrom age 16Time to learn: 20 minutes the first time, with your owner's manual; about 5 minutes after that

Park on level ground, read the oil dipstick and the coolant and brake fluid reservoirs against their marks, top up the washer fluid, and check the transmission fluid only if and how your owner's manual says.

SafetyNever open a radiator or coolant reservoir cap on a hot engine: hot coolant and steam can spray out under pressure and scald you. The electric cooling fan can start even when the engine is off, so keep hands, hair and loose clothing clear of it, and run the engine for a transmission check only outdoors, never in a closed garage. If brake fluid is very low or any level keeps falling, have the car inspected instead of topping up again.

Why it matters

Oil, coolant and brake fluid each do a job your car cannot do without, and their levels can fall between services: Ford notes that oil can drop a bit within the normal range between changes, and recommends checking it monthly. A level that keeps dropping between services is a reason to have a mechanic look for a leak. In NHTSA's 2005 to 2007 crash causation survey, the critical reason for a crash was assigned to the vehicle in about 2 percent of crashes, and brake problems made up about 22 percent of those.

How to do it

Park on level ground, set the parking brake and switch the engine off. Open the hood and match each cap and dipstick to the engine-compartment picture in your owner's manual, because positions vary from car to car. The engine oil dipstick sits near the engine and usually has a brightly colored handle.
Engine oil: wait until the engine has been off for at least 15 minutes, so the oil drains back into the oil pan; checking sooner, or on a slope, gives a false reading. Pull out the dipstick, wipe it with a clean, lint-free cloth, push it back in until it is fully seated, and pull it out again.
Read both sides of the dipstick and use the lower level. Between MIN and MAX, you need no oil. At or below MIN, add the oil your owner's manual specifies through the filler cap, in small amounts, and check again once it has drained down. Never add past MAX, never add oil to a hot engine, and never open the filler cap while the engine is running.
Automatic transmission fluid: look in the owner's manual before you look for a dipstick. Some cars have no owner check. GMC, for example, tells owners it is not necessary to check the fluid level, because a leak is the only reason fluid is lost, and says the job should be done at a dealer. If your manual gives no check you can do yourself, leave it to a dealer or mechanic.
If your manual does give a dipstick check, follow it exactly, because makers differ. In Mazda's version, the fluid is at normal operating temperature, about 150°F (65°C), the car is on level ground with the parking brake set, and the engine is idling. With your foot on the brake, move the shift lever through every position and back to P (Park). Pull out the dipstick, wipe it, put it back, pull it out again, and read both sides against the hot-range marks, between Full and Low.
Coolant: with the engine cool, read the level through the side of the see-through coolant reservoir. It should be between the MAX and MIN marks, sometimes labeled F and L. If it is at or below MIN, add the coolant your owner's manual specifies, up to MAX but not over. If you have to add coolant often, have the cooling system checked for leaks.
Never open the radiator cap or the coolant reservoir cap while the engine is hot, even after it is switched off: the system is under pressure, and hot coolant and steam can spray out and scald you. If you must open one, wait until the engine is cool, wrap a thick towel around the cap, turn it slowly counterclockwise to the first stop, and step back while the pressure escapes.
Brake fluid: read the level through the side of the brake fluid reservoir; it should be between MIN and MAX. It falls slowly as the brake linings wear, which is normal. If it is very low, or you have to add fluid often, have the brake system inspected. If you do add some, clean around the cap first, use only the brake fluid your manual specifies, never mix types, and keep it off the paint. If it gets in an eye, flush it with plenty of tap water and have a doctor check it.
Washer fluid: if the washer reservoir is low, fill it with windshield washer fluid. The lesson on changing wiper blades covers which fluid to use in winter.
Put every dipstick and cap back firmly, wipe up any spills, and close the hood until it latches. Check again about once a month, and if any level keeps dropping between services, have the car looked at.

Common mistakes

Checking the oil right after switching off the engine, or while parked on a slope, and adding oil the engine did not need.
Filling the oil past MAX, which can damage the engine.
Opening the radiator or coolant reservoir cap while the engine is still hot.
Checking transmission fluid cold or with the engine off when the manual says warm, idling and in Park.
Topping up brake fluid again and again instead of having the brakes inspected.

You can do this when …you can point to the oil dipstick and the coolant, brake fluid and washer reservoirs in your own car, read each one against its marks, and say from your owner's manual whether and how your transmission fluid is checked.