Under the bonnet: essential safety maintenance

To pass your driving test you need to Under the bonnet: essential safety maintenancecomplete the ‘show me, tell me’ vehicle safety questions. You’re required to answer two questions about your vehicle, pointing out where something is on your vehicle, or under the bonnet, and how to check these are safe and in working order.

Once many of us pass our tests though, we can be guilty of letting this engine maintenance knowledge fall by the wayside. Then, when we encounter problems with our cars, we’re not sure what to do. What compounds this situation further is that sometimes these engine repair jobs can be quite straightforward but with the right maintenance you can help to prevent vehicle issues from occurring.

So to help ensure you don’t end up looking like an idiot on the side of the road, AA Cars has detailed some essential under-the-bonnet safety maintenance checks. These cover how to find, check and, in some cases, top-up or fix any problems, covering a variety of different aspects and vehicle ages.


There are a number of important fluid reservoirs to be found under your bonnet that are essential to your car running safely and efficiently. These include:


Having the right oil level is vital, otherwise you can damage your engine at significant financial cost.

  • Park your car on a level surface, turn off the engine and leave it for at least half an hour to let the oil settle in your engine.
  • Open the bonnet and locate the dipstick (this is usually a bright colour and can feature an oil can symbol).
  • Pull this out, wipe off excess oil and insert it back into the reservoir.
  • Pull it back out and check if the oil level is within the right markers on the dipstick.
  • If not, unscrew the oil cap on the engine (again indicated by an oil can symbol) and with the appropriate oil (as indicated in your handbook) top up the oil.
  • Leave this to settle and then recheck the dipstick to see your new levels.

Brake fluid

Another important fluid, this ensures your hydraulic brakes are working properly.

  • With your car on a level surface and the engine turned off, open the bonnet and look for the reservoir. This is typically a small, semi-transparent, box-shaped feature found near the top of the engine.
  • You will be able to see the fluid through the reservoir and on the side there will be ‘MIN’ and ‘MAX’ level indicators. Simply check your fluid is in between these.
  • If it’s not, unscrew the cap and top up the levels with the right fluid – as detailed in your handbook.


To prevent your car from overheating and causing damage to its engine, you need to keep your coolant at the right level.

  • When the engine is off and cold, open the bonnet and find the reservoir above the radiator. This can be similar in shape and size to your brake fluid, so make sure this is the right one before you proceed.
  • Check the coolant levels are above the MIN and below the MAX levels indicated on the side.
    If necessary, unscrew the cap and top this up with an appropriate coolant (again as instructed in your handbook).
  • In some older vehicles you might not have a reservoir. If so, unscrew the radiator’s cap and top up until the coolant levels are up to just below this opening.

Washer fluid

Your washer fluid isn’t essential to your car’s engine running properly, but you must have some in your car to clear your front and rear windscreens if necessary.

  • Open your bonnet and look for the filler cap for the reservoir. Normally this is brightly coloured with a window washing symbol on it.
  • Open the cap and pour in your fluid – it’s best to use an already diluted fluid with anti-freeze mixed into it.
  • Fill this until the levels come up to just below the opening and replace the cap.


Along with your fluid levels, there are other important mechanical aspects to be found under the bonnet that you can check.


Both your timing belt and your drive belt are very important to your car. The former is a notched belt that connects your crankshaft and camshaft and times the opening and closing of your engine’s valves. The latter drives a number of important features of your engine, such as the various fluid pumps.

These can typically be found at the side of your engine (your manual can tell you where) and when your engine is off and cold you should check to see if these are frayed, split or corroded because your engine simply won’t work if they fail. A tell-tale sign of a damaged belt is a squealing or whining sound when your car is running.

Air filter

Your air filter protects your engine by stopping dust and debris from getting into it; however, this can become clogged and ineffective over time. It is typically located near the front or on top of the engine but in some vehicles it can be at the front of the engine bay (see your handbook).

It might need replacing altogether or could simply do with a quick clean. Either way, making sure the filter is in decent condition is a must to protect your engine.


It’s no secret that with a flat battery your car won’t work. If your vehicle is struggling to start or is sluggish, or certain controls or features aren’t performing as they should, it can be a sign that your battery is failing to charge.

Your battery can be found under the bonnet and is a large box with positive (+) and negative (-) terminals on top of it. You can jump-start your vehicle to recharge the battery; however, if this fails to work it could be a more complicated problem with your vehicle’s alternator.

For these more technical aspects though, it’s sensible to seek professional help to ensure these are fixed or replaced correctly. Otherwise, there is a risk that you could make existing problems worse and end up with an expensive repair bill.

As you might have gathered from this guide, your vehicle handbook is an essential tool in helping to ensure that your car is well maintained. So if you’re ever in any doubt, be sure to refer back to this for specific guidance and pointers regarding the right fluids, parts and locations of the above features.

Another top tip is to ensure your car is serviced annually as some of your fluid levels and certain mechanical parts will be changed or topped up. Otherwise, use this advice and get yourself up to speed with what’s going on under your car’s bonnet.


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