AdBlue: what the system needs and what stops it

A diesel engine burns with a large surplus of air and at high temperatures. That produces nitrogen oxides, and in oxygen-rich diesel exhaust an ordinary catalytic converter cannot break them down. The answer on most newer diesel cars is selective catalytic reduction, SCR for short: an aqueous urea solution is sprayed into the hot exhaust ahead of a dedicated catalyst.

In the heat the urea breaks down into ammonia, and in the SCR catalyst the ammonia reacts with the nitrogen oxides to form nitrogen and water. The process is monitored by NOx sensors, on many systems one before and one after the catalyst. From their readings the control unit works out how much to inject and whether the system is doing its job.

AdBlue is a brand name for this solution; the standard designation is AUS 32 under ISO 22241. It consists of high-purity urea and demineralised water, does not burn, but attacks some metals and leaves white crystals when it dries. Those two properties account for a large share of the typical faults.

Alongside the particulate filter, the SCR system is the second part of exhaust aftertreatment; on some engines the two are even combined in one component. But it works on a different logic: the filter collects and burns off, the SCR system doses continuously. So its faults are different too.

The tank is usually near the fuel tank or under the boot floor, with its blue-capped filler next to the diesel filler — on older models sometimes in the boot or the engine bay. Inside the tank sits a unit combining pump, level sender, heater and often a sensor that measures the quality of the fluid. From there a heated line runs to the injector in the exhaust.

Because the solution freezes at around minus eleven degrees Celsius, everything that carries fluid is heated. Freezing as such is allowed for and does no harm: after a start the heater thaws a supply, and after switching off many systems pump the line empty so nothing freezes in it. The humming you hear under the car after locking it is therefore usually not a fault. If a heater fails, though, it typically shows up in the first cold winter and not before. The typical weak points, one by one:

Wherever AdBlue meets the air and the water evaporates, urea is left behind as a hard white crust. That is harmless when it happens at the filler neck after topping up, and a warning sign when it turns up at a line connection, on the tank unit or around the injector mounting: fluid is escaping where it should not.

The second source is in the exhaust pipe. Urea only breaks down cleanly when the exhaust is hot enough. The control unit therefore only doses above a minimum temperature; but near that threshold, with lots of short trips and with an injector that no longer atomises finely, deposits still form on the pipe wall and on the mixer behind the injector. They grow slowly, narrow the passage and disturb the distribution until the sensors report insufficient conversion.

It is the same pattern as with the particulate filter: a diesel that is only driven briefly and cold never reaches the temperatures its exhaust system was built for. Regular longer drives help prevent it; crystals that have already set usually will not shift. Then the workshop has to clean or replace the injector and the pipe.

The third source is the fluid itself. AdBlue ages, especially when warm, and contaminated or diluted solution crystallises more readily and clogs filters and injector. So only fresh fluid from a sealed container goes into the tank.

Emissions rules require that a diesel with SCR cannot keep running indefinitely without a working system. On cars, manufacturers implement this in stages: as the supply runs low, a notice with the remaining range appears first, then a more insistent warning. Once the tank is empty the engine keeps running until you switch it off, but will not restart afterwards. So nobody grinds to a halt in the middle of the motorway — but you may well be stuck at a service area that has no AdBlue.

The same lockout is not triggered only by an empty tank. If the system detects a fault that prevents cleaning — a pump that does not deliver, an injector that does not dose, wrong fluid in the tank, an implausible sensor reading — a countdown in kilometres starts as well. Topping up does not clear this warning, and that is why an SCR message is not something to deal with later.

The right response depends on what the message says. If it is about the fluid level, top up before the range runs out. If it is about the system, you need a workshop appointment while the countdown is still running — not once the car is sitting in a car park and can only be towed. The instrument cluster shows how many kilometres are left; the owner's manual describes the stages and messages for your model.

After refilling a completely empty tank, many models require a minimum quantity and a little time with the ignition on before the new level is recognised and starting is allowed again. So if you are carrying a bottle, do not just pour in a splash.

The tank takes AdBlue or another solution to ISO 22241, and nothing else. No tap water to stretch it — the minerals in it form deposits that damage the pump and injector, and the quality sensor detects the dilution anyway. No additives, no leftovers from an opened container of unknown origin, no funnel that has previously been used for oil or screenwash.

The most expensive mistake is a mix-up. AdBlue in the diesel tank damages the injection system; diesel in the AdBlue tank damages the SCR system. The rule is the same either way: do not start the engine, ideally do not even switch on the ignition, and have the car taken to a workshop that will drain the tank and flush the system. Every minute the wrong fluid is pumped around adds to the damage.

Many filling stations have AdBlue pumps with a nozzle for cars; otherwise you fill from bottles or containers with a matching spout that prevents overflow. Do not stockpile: the solution keeps only for a limited time, and the warmer it is stored, the shorter that time. The boot in summer is the wrong place for it. And if some gets spilled, wipe it up with a damp cloth before it crystallises.

How much an engine uses depends on the engine, the load and the driving; a rule of thumb is of little help. More reliable is the remaining range in the instrument cluster, watched over a few weeks. A sudden, marked rise in consumption with no change in driving belongs in a workshop — and so does consumption close to zero on a diesel that has the system.

With a used diesel with SCR there are three things to establish: whether the system is working without faults right now, whether it has had problems in the past, and whether it is still in original condition. The first is shown by a test drive long enough for the exhaust to get properly hot, followed by a diagnosis that also reads the exhaust system's entries. Stored faults without an active warning are the more interesting part.

The second is shown by the invoices: replaced NOx sensors, a new injector, a tank unit, a cleaned exhaust pipe. A single replacement is normal. Several jobs on the same part in quick succession point to a cause that was never found, often a driving pattern heavy on short trips. Ask too about manufacturer software campaigns for the exhaust system; a franchised workshop can list outstanding ones from the VIN.

The third is trickier. There are devices and software that disable the SCR system and make the control unit believe it is working. Such modifications are not permitted on public roads, and whoever buys a car altered this way inherits the problem, at the latest at an inspection or a roadside check; the consequences depend on the country. Signs include an AdBlue level that has not moved for months, boxes or plugs added to the sensor wiring, and a seller who says he has had “no more trouble” with AdBlue.

Also look around the filler neck, the tank unit and the injector: white crusts at connections mean a leak. And ask not only about warning lights but about messages — an SCR warning often appears as text on the display. On Carvexia, “Is any warning light on in the dashboard?” is one of the questions in the seller's statements; the answer is the seller's statement, not checked, and it replaces neither a diagnostic scan nor your own look at the display with the engine running.