Mercedes-Benz GLA driving

What is AdBlue and does every diesel car need it?

Many of the latest diesel cars use a fluid called AdBlue in addition to fuel. But what exactly is AdBlue? Here’s everything you need to know.

Graham King Cazoo

By Graham King

Updated: 4 October 2023

Many modern diesel cars that meet the latest emissions standards (called Euro 6) use a fluid called AdBlue, which helps remove toxic substances from the car’s exhaust gases. But what is it? Does your car – or one that you're considering buying – need it? Read on to find out.

What is AdBlue?

AdBlue is a liquid added to many diesel cars that reduces the harmful emissions they can create. AdBlue is actually a brand name for what’s technically known as diesel exhaust fluid. It’s a solution of distilled water and urea, a substance found in urine and fertilisers. It’s non-toxic, colourless and has a slightly sweet smell. It feels a bit sticky if you get any on your hands but it washes off easily.

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Why does a diesel car need AdBlue?

AdBlue helps to reduce emissions of nitrogen oxides, or NOx, from a car’s exhaust. Euro 6 emissions standards, which apply to all cars built since September 2015, place strict limits on the amount of NOx that can be emitted legally. These NOx emissions are a waste product of the combustion process – the burning of a mixture of fuel and air inside an engine – that produces the power to drive a car.

Such emissions have been linked to respiratory conditions that can seriously affect people’s health. NOx emissions are a key measurement for deciding which cars can enter a clean air zone, such as London’s Ultra Low Emission Zone (ULEZ), without paying a fee. Diesel cars registered after September 2015, which meet Euro 6 standard, are ULEZ-compliant and won’t be charged for entering.

How does AdBlue work?

AdBlue is used as part of a car’s selective catalytic reduction, or SCR, system and is automatically injected into your car’s exhaust system where it mixes with the exhaust gases, including the NOx. The AdBlue reacts with the NOx and breaks it down into harmless oxygen and nitrogen that comes out of the exhaust pipe and disperses into the atmosphere.

AdBlue doesn’t eliminate all of your car’s NOx emissions, but it does greatly reduce them.

How much AdBlue will my car use?

There’s no set rate at which cars use up their AdBlue. In most cases, it takes several thousand miles to empty the car’s AdBlue tank. Some can do at least 10,000 miles before needing a refill. It’s also worth noting that, contrary to some reports, using AdBlue doesn’t mean you’ll burn more fuel.

How do I know how much AdBlue is left in my car?

All cars that use AdBlue have a gauge or display somewhere on the dashboard or in the trip computer showing how much is left. Consult your owner’s manual to find out how to view it. A warning light will show on the driver’s display long before the AdBlue tank becomes empty.

Can I refill the AdBlue myself?

Not every car allows you to refill the AdBlue tank yourself, but you can easily find out if yours does. Behind the fuel filler flap, there’ll be an extra filler with a blue cap for the AdBlue, next to the regular diesel filler. The tank itself is underneath the car, next to the fuel tank.

AdBlue is available from most fuel stations and car parts shops. It comes in containers of up to 10 litres, which typically cost between £15 and £20. The container will come with a nozzle to make pouring the AdBlue into the filler much easier. Alternatively, fuel station HGV lanes have AdBlue pumps that you can use to fill your car if it has a suitable nozzle.

What happens if I put AdBlue in a diesel tank?

It’s absolutely vital that you don’t accidentally pour AdBlue into your car’s fuel tank. If you do, the tank will need draining and flushing clean. Fortunately, you can’t put diesel in the AdBlue tank by accident because the pump nozzle is far too big.

If your car doesn’t have a dedicated AdBlue filler, the tank can only be refilled by a garage (because the filler is usually hidden below the boot). The tank should be topped up every time your car is serviced, so make sure the garage doing the work includes it. If the tank needs refilling between services, most garages will do so for a small charge.

What happens if my car runs out of AdBlue?

You should never let your car run out of AdBlue. If it does, the engine will go into ‘limp’ mode, which drastically reduces power to keep NOx emissions within legal limits. If that happens, a warning will appear on the driver’s display and you should refill the AdBlue tank as soon as possible. You should also avoid turning off the engine until you have access to more AdBlue, because the engine is unlikely to restart.

Incidentally, running out of AdBlue is just one of many reasons that an engine will go into limp mode. Any severe problems with the engine or gearbox that occur while driving will activate limp mode. It’s there to prevent further damage and keep the car moving so you can stop in a safe place to call a breakdown service, or quickly get to a garage.

How to reset an AdBlue warning

If your AdBlue runs low and needs refilling, your car will flash up an alert – either on the driver’s display or on the infotainment system’s screen. Once you’ve filled the AdBlue tank to the recommended level, the warning should disappear. If it doesn’t, try driving for a few miles so that the system resets.

If after 10 miles or so the warning still doesn’t disappear, there could be a problem with the system or the sensors that it uses. You may be able to find some guidance in your car’s manual on what to do in this case; it’ll vary depending on the make and model. If the manual is unclear, or the recommended steps have no effect, then the best option is to take your car to a mechanic.

Which cars use AdBlue?

Many diesel cars that meet Euro 6 emissions standards use AdBlue. However, not all of them do, as there are other systems that can be used instead to reduce NOx emissions.

Many cars use AdBlue, so here are some clues to help you figure out whether the car you want to buy uses AdBlue:

  • Check whether the word ‘blue’ or the letters ‘SCR’ are part of the car’s name. For instance, Peugeot and Citroen diesels that use AdBlue are badged BlueHDi. Fords are badged EcoBlue. Volkswagens are badged TDI SCR.

  • Open the fuel filler flap to see whether there’s the blue-capped AdBlue filler mentioned earlier. If you’re still unsure, ask the dealer or the manufacturer.

Is AdBlue harmful to the environment?

No. AdBlue isn’t harmful to the environment. It’s not toxic and it isn’t flammable. If you spill it, you can just wipe it up or dilute it with water and mop it up.

We wouldn’t recommend touching it, however, because it can irritate your skin. It can also stain clothing and can be mildly corrosive on some materials, including certain metals, plastics and your car’s paintwork. So if you spill any on your car, wipe it up as quickly as you can.

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