What does that warning light mean and what should you do?

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2. What does that warning light mean and what should you do?

As cars have become more complex, the number of different dashboard warning lights has grown significantly. Modern electronic systems like airbags or stability control are constantly self-testing and it is important for you to know whether a warning light requires immediate attention or whether you can continue your journey and get a garage to investigate later.

If a warning light comes on (or fails to turn off after you've started the engine), you should stop as soon as possible in a safe place.

Many warning lights are specific to the car or even the model — the vehicle handbook will give you a detailed explanation of their meaning and the action to take.

MOT tests

Some mandatory test items were introduced into the MOT test in January 2012. These include checks for the correct function of the following warning lights (where fitted):

• Electronic parking brake
• Electronic stability control
• Headlight main beam
• Electronic power steering
• Brake fluid level
• Tyre pressure monitoring system
• Airbag
• Seatbelt pre-tensioner

Oil pressure warning light

This light should illuminate when the ignition is switched on and should extinguish as soon as the engine starts.

If the light stays on after starting, or illuminates during a journey, stop immediately, switch off the engine and check the engine oil level. Top it up straight away if the level is low.

If the warning light illuminates even though the oil level is correct, do not start the engine and seek assistance.

Battery charge warning light

This light should illuminate when the ignition is switched on and should extinguish as soon as the engine starts.

If it does not illuminate at all, or if it illuinates while driving, your battery is not being charged as there is a fault with your charging system.

This may be due to:

• a slack battery or starter terminal
• a broken or loose alternator drive belt
• an alternator failure

If the drive belt is broken, it must be replaced before you restart the engine. The coolant system may rely on this belt and its failure could cause the engine to overheat, in turn causing engine damage.

Immediately move the vehicle to a safe location and switch off the engine. Do not restart the engine and seek assistance.

Brake system warning light

This light is most commonly seen when the handbrake is on. Illumination after releasing the handbrake may indicate low brake fluid level.

Refer to the vehicle handbook and add the correct brake fluid at once to bring the level up to the MAX mark. As the brakes wear, the fluid level will slowly drop. Check your brake fluid level frequently to ensure there is no further rapid loss of fluid.

If the brake fluid is too low and the brake pedal travel is distinctly longer than usual, one of the two hydraulic brake circuits may have failed. Do not continue your journey and seek assistance.

If the light remains illuminated, even with the handbrake off and the correct brake fluid level, this may indicate a sensor fault. Take the vehicle to a dealer or garage as soon as possible.

Diesel Particulate Filter (DPF) warning light

DPFs catch bits of soot in the exhaust. As with any filter (think of the bag in a vacuum cleaner), they have to be emptied regularly to maintain performance. For a DPF, this process is called regeneration — the accumulated soot it burnt off at high temperature leaving only a tiny ash residue. Regeneration may be either passive or active — see here for more information on DPF regeneration.

It should be possible to start a complete regeneration and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph. If the warning light does not extinguish after this, take your vehicle to a dealer or garage as soon as possible. Ignoring the DPF warning light can cause irreperable damage to the filter.

Engine warning light

This light should illuminate when the ignition is switched on and should extinguish as soon as the engine starts.

If it illuminates with the engine running, it indicates a malfunction with the engine management system. Have this checked as soon as possible.

If it flashes when driving, reduce the vehicle speed immediately until the light stops flashing and stays constantly lit. If it continues to flash, avoid heavy acceleration and high engine speed, stop the vehicle as soon as safely possible, and have it checked immediately.

On some vehicles this light may be red — again, stop the vehicle and seek assistance.

If it is safe to do so, try stopping the vehicle and switching off the engine. Wait two minutes, then resart the engine to reset the engine management system.

With the light illuminated, the vehicle is still safe to drive as long as no other faults are apparent with the engine. But have the vehicle checked by a dealer or garage as soon as possible to avoid any damage being caused to the catalytic converter.

ABS warning light

When the ignition is switched on, this light illuminates briefly to confirm that the system is operational.

If it illuminates when driving, it indicates a malfunction. Have this checked as soon as possible.

Normal braking (without ABS) will generally be maintained and many cars will be safe to continue driving, but some will not. Check the vehicle handbook for advice.

Brake system & ABS warning lights together

If both warning lights illuminate at the same time when driving, stop the vehicle as soon as it is safe to do so.

Reduce vehicle speed gradually and immediately move the vehicle to a safe location. Use the brakes with great care. Do not step on the brake pedal abruptly. Seek assistance.

Airbag warning light

When the ignition is switched on, this light illuminates briefly to confirm that the system is operational.

If it remains on or illuminates when driving, it indicates a malfunction. The airbags are an important part of your car's impact protection, so have this checked as soon as possible.

Coolant warning light

When the ignition is switched on, this light illuminates briefly to confirm that the system is operational.

If it illuminates when driving, it indicates that the car's engine temperature is too high. Damage can occur if you continue to operate the engine with excess temperature.

Pull over when it is safe to do so, and check your car's coolant level using the gauge on the coolant tank under the bonnet, topping up if necessary.

If the coolant temperature gauge reading is also well into the red, this could mean your engine is overheating. This is either the sign of a larger problem — like a head gasket failure — or symptomatic of something less major, like a leak in the coolant somewhere. Take the vehicle to a dealer or garage as soon as possible to avoid any damage.

Tyre pressure warning light

More and more cars are fitted with type pressure monitoring systems these days, and not just high-end expensive vehicles.

These systems can sense a deviation from normal tyre pressures, which may signify a puncture. If this light illuminates, pull over when it is safe to do so and check your vehicle's tyres to ensure one or more is not flat or in the process of deflating with a slow puncture. If a visual check does not indicate a problem, you may need to check the tyre pressure using a pressure gauge (many service stations have an electronic gauge you can use if you don't own a pen gauge).