Mercedes AdBlue warning: meanings and what to do

An AdBlue warning on a Mercedes-Benz diesel means one of two things: the AdBlue tank is running low, or the system that uses it has a fault. A low level message is solved by topping up with AdBlue, and you should do it soon, because once the countdown on the display reaches zero the engine will not start again until you do.

A warning that stays on after a top up, or arrives with the engine warning light, points to a fault that needs a diagnosis. This guide explains what each Mercedes message means, how to top up correctly, what causes AdBlue faults and how they affect the MOT.

What AdBlue does in a Mercedes diesel

AdBlue cleans the exhaust. Mercedes-Benz diesels with selective catalytic reduction, which Mercedes badged BlueTEC, spray a small amount of AdBlue into the exhaust, where it reacts in a catalyst and turns most of the nitrogen oxides into nitrogen and water vapour. Diagnostic equipment maker Snap-on puts the reduction at up to 90 per cent.

AdBlue itself is 32.5% high purity urea mixed with purified water. It is not an additive and it never goes in the fuel tank. It freezes at around minus 11°C, and a fleet guide from Arval puts consumption at about five litres of AdBlue for every 100 litres of diesel, with car tanks typically holding 18 to 30 litres. On many models the AdBlue filler sits next to the diesel filler, behind the same flap, but some earlier cars put it elsewhere, so check the AdBlue section of your owner's manual.

How often you will need to top up

It depends on how far you drive and how much diesel you burn, because AdBlue use follows fuel use. Arval's rule of thumb of five litres of AdBlue for every 100 litres of diesel gives a rough guide: a car averaging 45mpg covers about 990 miles on 100 litres of diesel, so it would use around five litres of AdBlue in that distance.

Many owners rarely see the reserve message at all, because the tank is often topped up during routine servicing. If you cover high mileages between services, keep a container in the garage at home so a warning on a Sunday evening does not become a problem on Monday morning.

What each Mercedes AdBlue message means

The wording depends on the age of the car. Older models count down in miles, while newer models with the MBUX system can also limit speed before the engine is stopped from starting. These are the messages Mercedes-Benz lists in its owner's manuals:

The key point is the countdown. A Mercedes will keep driving while the miles or kilometres count down, but once the figure reaches zero and you switch off, it will not restart until AdBlue goes in. If the message appears on a long journey, top up before you stop for the night.

How to top up AdBlue on a Mercedes

Mercedes-Benz sets out the method in its owner's manuals, and it takes a few minutes:

1. Open the fuel filler flap, or find the AdBlue filler your manual shows, and unscrew the AdBlue cap anticlockwise. Make sure you have the AdBlue filler, not the diesel filler.

2. Fit the container's hose to the AdBlue filler neck, finger tight, or use an AdBlue pump at a filling station.

3. Lift and tip the container. Filling stops automatically when the tank is full, so do not try to force more in.

4. Refit the cap, close the flap and store any part used container outside the car, because a leak can contaminate the interior.

5. Switch the ignition on for at least 60 seconds before starting the engine, so the car can detect the new level. The message should then go out.

Buy AdBlue from a sealed container or a pump, and keep it clean. Most UK filling stations and motor factors sell it.

If AdBlue goes into the diesel tank, do not start the engine. Mercedes warns that AdBlue in the fuel causes engine damage. The tank needs draining before the car is driven, so call for help where the car stands.

When an AdBlue warning means a fault

Some AdBlue messages have nothing to do with the level in the tank. If the warning comes back after a correct top up, the countdown carries on, or the engine warning light comes on with it, the system has found a fault.

Snap-on's technical guide to selective catalytic reduction lists the usual problem areas:

• NOx and temperature sensors before and after the catalyst, which tell the system whether it is working.

• A blocked injector in the exhaust that sprays the AdBlue.

• Low AdBlue pressure from the pump or the air supply. Snap-on warns that confusing these two causes is the most common misdiagnosis on these systems.

Because several faults produce the same message, replacing parts on a hunch gets expensive. A diagnosis reads the stored fault codes, checks the sensor readings and pressures, and finds the part at fault before anything is ordered.

Does an AdBlue fault fail the MOT?

It can. An AdBlue leak on its own is not a reason to fail, as the MOT inspection manual specifically excludes fluid for selective catalytic reduction from the leak check. But on diesel cars first used from 1 July 2008, an engine malfunction indicator lamp that is lit or shows a fault is a major defect, and an AdBlue fault can turn that lamp on.

If an AdBlue warning or the engine light is showing in the weeks before your MOT, get it diagnosed first. Our MOT checklist lists the other dashboard warnings that fail the test.

Common AdBlue mistakes to avoid

• Ignoring the reserve message. It gives you warning, but the countdown does not stop for a weekend away.

• Topping up with the wrong fluid. Water, screenwash or additives do not replace AdBlue.

• Overfilling. The tank stops itself when full, and Mercedes warns against overfilling.

• Keeping spare AdBlue in the cabin. A leaking container can contaminate the interior, so store it outside the car.

• Clearing the warning without fixing the cause. A fault message returns until the fault is repaired.

Mercedes AdBlue warning questions

How far can I drive with the AdBlue warning on?

Until the distance shown on the display reaches zero. The engine keeps running, but once you switch off at zero it will not restart until you top up. On newer models, performance and speed can be limited before that point.

How much AdBlue does my Mercedes need?

Newer models display the minimum quantity to add. Otherwise, fill until the tank stops accepting fluid. Car AdBlue tanks typically hold between 18 and 30 litres.

Why is the AdBlue warning still on after I filled it up?

Switch the ignition on and wait about 60 seconds before starting, because the car can take that long to detect the top up. If the message still shows, or the countdown continues, the system has a fault that needs diagnosing.

Can I reset the AdBlue warning myself?

A low level message clears itself once you add enough AdBlue and the car detects it. A fault message needs the fault repaired, and clearing the code without a repair only brings it back.

Does AdBlue freeze in winter?

AdBlue freezes at around minus 11°C. Store spare containers somewhere frost free, and keep them sealed so the fluid stays clean.

Mercedes AdBlue diagnosis near High Wycombe

Wheels VT is an independent Mercedes-Benz specialist at Southside Farm, Wooburn Green, High Wycombe. If your AdBlue warning will not clear, we read the fault codes, find the cause and explain the repair and its cost before any work goes ahead. Computer diagnostics start from £78 plus VAT, and Mercedes servicing starts from £100 plus VAT.

See our Mercedes-Benz servicing page, call 01494 670922 or book online. For the service codes on your dashboard, read our guide to Mercedes Service A and Service B.