CitiRental | Car Warning Lights
The Complete Guide

OIL WARNING LIGHT Aside from a brake warning light, an oil warning light is one of the most severe warning lights of all; hence it is red. Oil pressure is generated by the oil pump keeping the engine oil circulating and by the quantity of oil in the engine. A leak, a faulty pump, or too little (or even too much) oil can cause damage to the engine, very likely resulting in huge repair bills or even a write-off.

ENGINE MANAGEMENT WARNING LIGHT If this warning illuminates, it’s very important to get the car checked, even if it feels completely normal to drive. This is because it warns of a potential problem with the engine, which you’ll want to avoid damaging at all costs. This light can come on for a very wide range of problems, so while it’s likely to be just a faulty sensor or minor issue, a proper diagnosis is critical to prevent potential damage.

BRAKES Another red warning, this brake system warning light also requires urgent attention. It may be that you’ve left the handbrake engaged slightly while pulling away, so ensure it’s fully released – some cars may instead have the letter ‘P’ instead of an exclamation mark to represent that the parking  brake is on. If this doesn’t solve the problem, or the warning light appears as you’re driving along, pull over safely and call for roadside assistance or arrange for the car to be taken to a garage. It may be that the brake fluid level is too low and simply needs topping up or that a brake pad wear sensor is faulty. However, it could also be a more serious issue with the braking system. While you can’t take the risk either way, if the pedal feels spongy when you press it or sinks to the floor, it’s easy to tell that an issue has occurred, and the car should not be driven.

BATTERY WARNING This warning light can indicate a number of things, all relating to the car’s battery and charging system. This is a particular problem at night because a faulty battery can lead to headlights failing, but without the battery or charging system providing electricity, you may find power steering, brake servos or the engine itself stop working. The cause of the warning light may be as simple as a faulty battery that’ll need replacing. But it could also be due to a problem with the car’s wiring or, more seriously, a fault with its alternator or drive belt. Other signs of a failing battery could be dim headlights or an engine that’s reluctant to start. Most garages can check your car’s battery and charging system to identify what’s at fault. Find out how to change your car’s battery.

TEMPERATURE [OVERHEATING] WARNING This one is self-explanatory. If you see the light on at any time after start-up, then the engine is running too hot or there isn’t enough coolant (also called antifreeze) in the system. This may be due to several issues, all of which could leave you stranded in a cloud of steam. It could be that the car’s radiator is leaky, clogged or broken – you’ll likely see coolant dripping out if this is the case. It could also be due to a faulty water pump. More serious is a blown head gasket, which can signal catastrophic failure if you don’t turn off the engine immediately. White smoke from the exhaust could be a sign of head gasket failure. Some cars may also have another one of these lights but blue. This light means that your engine is cold, so it should appear when you are starting your car after it has been sat for a while. The blue light isn’t one to worry about but it’s advisable not to push your engine too hard until it has disappeared.

LOW FUEL WARNING This is self-explanatory, but we’ll go through it anyway. This is a warning light that comes on when the amount of fuel in the tank has passed a certain level. It’ll usually come on when you’ve got around 50-70 miles of range and is designed to give enough warning so you can find a fuel station. Running out of petrol won’t cause any mechanical damage but stopping on the road due to running dry is an offence. If you run out of diesel, it’ll be necessary to bleed the air from the fuel lines and injectors before the engine starts properly. That means it’s more than simply topping up from a jerrycan. If you’re using more fuel than you’d expect, it could mean that you have a fuel leak (check the ground under your car; especially your driveway or parking space), or your car’s on-board computer (ECU) is telling the engine to use more fuel than it needs. A garage will be able to check this for you.

TYRE PRESSURE WARNING Tyre-pressure monitoring systems (TPMS) are an increasingly common sight; indeed, for the last few years, it’s been mandatory for carmakers to fit the system. Essentially, it constantly monitors the amount of air in the tyres. If it detects a drop, it’ll provide a warning. This is important because low tyre pressures can adversely affect braking and cornering, and a sudden drop in pressure can cause a dangerous blow-out at high speed. If the TPMS system alerts you to low pressure, you’ll need to check and inflate the tyre. If it has an obvious puncture, you’ll need to change the wheel or use a puncture repair kit.

GLOW PLUG WARNING (DIESELS ONLY) Unlike petrol cars, diesel-powered vehicles don’t need a spark to make the air/fuel mixture go ‘bang’ and get the whole engine running. Instead, they use compression and temperature to induce combustion in the mixture. Every diesel has ‘glow plugs’ that are used to turn up the heat and the above symbol indicates that something’s wrong with the plugs themselves or the systems and computers that control them. You may not notice anything wrong at first, but issues with glow plugs can induce early and/or late combustion (known as ‘knocking’). This will make your engine sound a bit rough and not run as smoothly or efficiently as it should, resulting in a drop in fuel economy.

SEAT BELT WARNING These days, most new cars come with a system to let you know if any of your passengers haven’t got their seatbelts fastened. There’ll usually be a light – as above – to tell you if this is the case. Cars use pressure sensors in the seats and sensors in the seatbelts – so if there’s a certain amount of weight on a seat and the seatbelt isn’t secured, it’ll let you know. These warnings may be accompanied by an unusual noise, too.

BATTERY CHARGE LEVEL WARNING This light will appear if your battery is at a critically low level of charge. If you do not stop the vehicle soon, you could end up stranded. The best and most obvious, course of action is to seek the nearest available charging point, whether this is at home or a public facility.

DOOR/BOOT/BONNET WARNING Like the seatbelt light, these lights don’t usually mean that there’s anything physically wrong with the car (unless the sensor that sends the signal to the light is itself faulty). Instead, they tell you that one of the openings – be it a door, boot or bonnet – is open. This is often obvious if, for example, you’re putting the weekly shop into the boot or securing your children in the back seats. But these lights also tell you if a door, boot, or bonnet hasn’t been shut properly and is only loosely closed so that they can be very useful.

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