Safety: only do jobs you are confident with. Never work under a car supported only by a jack, and if a repair affects brakes, steering or suspension and you are unsure, have a garage do it or check your work.
Checking your fluids every month or two is the simplest maintenance there is, and it prevents some of the most expensive faults. Leaks also matter at the MOT: in 2025, 3.9% of car MOT tests failed on noise, emissions and leaks, which includes oil or fuel leaks and excessive exhaust smoke.
Safety first
- Park on level ground with the engine off. Check oil after the engine has been off for at least five minutes, so the oil has drained back down.
- Never open the coolant cap on a hot engine. The system is under pressure and can spray scalding coolant. Wait until the engine is cold.
- Keep hands, hair and clothing clear of the cooling fan: on many cars it can start even with the engine off.
Engine oil
- Find the dipstick: usually a yellow or orange loop handle.
- Pull it out, wipe it clean, push it fully back in, then pull it out again.
- The oil should be between the two marks (MIN and MAX, or two dots). Look at the colour too: fresh oil is amber; very black, gritty oil is overdue for a change.
- To top up, unscrew the filler cap on top of the engine (it often has an oil-can symbol) and add the grade shown in your handbook, such as 5W-30. Add a little at a time and recheck. Don’t overfill; too much oil can damage the engine and catalytic converter.
Some newer cars have no dipstick and show the level on the dashboard instead. If you are topping up more than about a litre between services, the engine is burning or leaking oil and is worth getting checked.
Warning signs: milky, mayonnaise-like residue under the filler cap can mean a head-gasket problem, though short journeys in cold weather can cause a little. An oil pressure warning light while driving means stop the engine as soon as it is safe.
Coolant
- Find the coolant expansion tank: a translucent plastic bottle with MIN and MAX marks, usually with a pressure cap.
- With the engine cold, check the level is between the marks.
- To top up, undo the cap slowly and add the coolant type specified in the handbook, mixed with water as directed. Coolants are different colours and types, and some shouldn’t be mixed.
Coolant that keeps dropping means a leak. Look for drips under the front of the car, crusty deposits on hoses, or a sweet smell after driving. White smoke from the exhaust that smells sweet is a sign of coolant getting into the engine.
Brake fluid
Check the level is between MIN and MAX in the reservoir near the driver’s side of the bulkhead. Don’t top up with anything other than the specified fluid (usually DOT 4), and don’t open the cap unnecessarily, as the fluid absorbs moisture from the air. A level that drops noticeably means a leak or very worn pads. See our brake guide.
Power steering fluid
Many modern cars have electric power steering with no fluid. If yours has a reservoir (often marked with a steering-wheel symbol), check the level against the marks with the engine off. Whining when you turn the wheel can mean the level is low.
Screenwash
An empty washer bottle is an MOT failure, so keep it topped up with proper screenwash. See our guide to wipers and washers.
Finding a leak
Leave the car overnight with a sheet of cardboard under the engine. The colour of any drips tells you what is leaking:
- Brown or black, oily: engine oil
- Green, pink, orange or blue, slightly sticky and sweet-smelling: coolant
- Clear to yellow, slippery: brake fluid. Get this checked before driving further.
- Red: automatic gearbox or power steering fluid
- Clear water under the front passenger area: usually just air-conditioning condensation, which is normal.
At the MOT, an oil leak fails if it is dripping significantly or could cause a hazard, so a clean engine bay and a fixed leak avoid a fail.
Which cars fail on emissions and leaks most?
From our analysis of every car MOT in 2025, these popular models failed on emissions and leaks most often. If you own or are buying one, it is worth checking this area more carefully. Older cars naturally fail more, so the figures partly reflect each model’s age.
| Model | Tests failing on emissions and leaks | Compared with average (3.9%) |
|---|---|---|
| MINI MINI | 15.7% | 4.0× |
| Volkswagen Fox | 15.3% | 3.9× |
| Fiat Punto | 12.9% | 3.3× |
| Volkswagen Beetle | 12.2% | 3.1× |
| Peugeot 206 | 11.9% | 3.1× |
| Peugeot 208 | 11.1% | 2.9× |
See every model in our MOT data by model.
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