When AdBlue® problems reach a workshop, they usually arrive as a warning light or a truck stuck in limp mode. The cause, though, often goes back weeks or months to what went into the AdBlue tank. Understanding how bad product damages an SCR system helps explain why low-grade or poorly stored fluid can become an expensive repair.
How the SCR system works
AdBlue is drawn from its own tank (usually with a blue cap, separate from the diesel tank) by a supply module containing a pump and a fine filter. It is pumped to a dosing injector that sprays it into the hot exhaust. There it turns into ammonia, which reacts across the SCR catalyst to convert NOx into nitrogen and water. NOx sensors before and after the catalyst tell the engine computer how well it's working, and many systems also have a quality sensor in the AdBlue tank.
Every one of those parts is designed around ISO 22241 fluid: 32.5% urea, and almost nothing else.
What each type of bad AdBlue does
Diluted AdBlue
Adding water, or rainwater getting into a tank, drops the urea concentration. The system doses less ammonia than it expects, NOx conversion falls, and the downstream sensor reports high NOx. The result is usually a quality or emissions warning and, if it continues, a power derate. Diluted AdBlue also freezes at a higher temperature than the −11°C of correct product.
Tap water and minerals
Tap water adds calcium, magnesium and other minerals that ISO 22241 limits to 0.5 mg/kg each. They don't burn off in the exhaust. Over time they build up as deposits on the injector tip and on the catalyst surface, reducing its active area. This damage is gradual and generally can't be reversed by cleaning.
Metals from incompatible equipment
AdBlue is corrosive to carbon steel, aluminium, copper, brass and galvanised steel. Fittings, pumps or containers made of those metals release copper, zinc, iron and aluminium into the fluid. Along with the discolouration and sediment you might see, dissolved metals act as catalyst poisons, coating the active sites so the catalyst no longer converts NOx properly.
Particles and dirt
Dust through open lids, dirt on a dropped nozzle and debris from reused containers end up in the vehicle's supply module filter. A loaded filter starves the pump, dosing pressure falls and the system flags a fault. Fine particles that get past the filter can wear the pump and partly block the injector, causing a poor spray pattern and uneven dosing.
Diesel, oil or coolant
This is the most serious. Hydrocarbons are not compatible with the seals and materials in the AdBlue system. They can cause seals to swell and fail and can damage the pump, sensors and lines. Contamination of this kind often means the whole AdBlue system has to be drained and flushed, with components replaced. If diesel or oil is found in an AdBlue tank, don't keep running the vehicle; talk to your workshop.
Degraded or overheated AdBlue
AdBlue stored hot for too long breaks down, releasing ammonia and forming more biuret. The concentration drifts, and the by-products can contribute to crystalline deposits in the injector and exhaust. A strong ammonia smell from a container is the warning sign.
The symptoms you'll see
- AdBlue quality, SCR or emissions warning lights
- Reduced engine power (derate or limp mode), and on some vehicles a restart limit
- Blocked SCR filters found at service
- Injector blockages or white crystalline deposits in the exhaust
- NOx sensor faults, which can be the sensor or a sign of poor conversion
- Several vehicles with similar faults after filling from the same source
Repairs can range from a filter change to replacing the dosing module, sensors or the catalyst itself, and the vehicle is off the road while it happens. That downtime is often the bigger cost for a fleet.
Preventing it
- Buy AdBlue made to ISO 22241 by a VDA-licensed producer, and ask for the batch Certificate of Analysis.
- Use dedicated equipment in compatible materials: stainless steel, HDPE, polypropylene and EPDM.
- Never top up with water or reuse containers that held other products.
- Store out of direct sun, and keep lids and valves closed.
- Check concentration with a refractometer if in doubt.
One thing to do today: label every funnel, jug and hose used for AdBlue clearly as "AdBlue only", and move anything used for diesel, oil or coolant to a different shelf. Mixed-up equipment is one of the most common ways hydrocarbons get into an AdBlue tank.
How Liquidia can help
Liquidia manufactures VDA-licensed, ISO 22241 compliant AdBlue with every batch lab-tested and a 12-month product guarantee. Read what AdBlue is and how it works, or see why keeping the SCR system healthy pays off on our emissions value page.