AdBlue is not an inert puddle. The manufacturer’s technical data sheet for the fluid is explicit that non-alloyed steels, zinc-coated steels, copper and copper alloys are unsuitable for contact with it, because they stand up poorly to urea solution and the ammonia dissolved in it. That line exists to tell engineers what to build a tank out of. It is also the reason an AdBlue leak under your van deserves more attention than the size of the puddle suggests.
Most guides on this subject tell you AdBlue is harmless because it is non-flammable and non-toxic. Both of those things are true, and both of them miss the point. The problem with a leak is never the fluid on the floor. It is what the fluid is doing to the bracket, clip, connector or sender flange it is creeping across on the way down — and the fact that a system losing fluid is a system that will eventually stop dosing correctly and put a countdown on your dashboard.
What does an AdBlue leak look like? A fresh AdBlue leak is a thin, clear or faintly yellow puddle under the middle or rear of the vehicle, with a sharp ammonia smell. As the water in it evaporates, it dries to a white or off-white crust around the joint it escaped from. That crust is crystallised urea, and it marks the exact point the fluid is getting out.
In this guide
- How to tell AdBlue from every other fluid under your vehicle
- The six places an AdBlue leak usually starts
- Why the white crust is the diagnosis, not the mess
- What a leak does to the rest of the system if you leave it
- What a proper repair involves
- Frequently asked questions
Is it actually AdBlue? How to tell it apart
Before you chase an AdBlue leak, make sure that is what you have. Three fluids get confused constantly, and they point at completely different repairs.
AdBlue — clear to very slightly yellow, thin and watery, not oily to the touch. Strong ammonia smell. Dries to a white powdery or crusty deposit. Comes from the middle or rear of the vehicle, near the tank, the pump module or the exhaust.
Coolant — coloured (pink, blue, orange or green), noticeably slippery, sweet smelling. Usually from the front of the vehicle. Leaves a coloured stain, not a white crust.
Screenwash or condensation — condensation from the air conditioning is plain water, odourless, and evaporates leaving nothing behind. If a clear puddle dries to nothing at all on a warm day, that is almost certainly what it was.
The ammonia smell and the white residue together are close to conclusive. AdBlue is an aqueous urea solution — the standard it is made to, ISO 22241, specifies a urea content of 31.8 to 33.2 per cent by mass. Roughly a third of what leaks out is solid urea. When the water evaporates, that third stays behind as crystal.
Where an AdBlue leak usually starts
On the vans and cars we see most, leaks cluster in six places. Knowing which one you are looking at changes the repair completely.
1. The tank sender flange
The level sender and heater assembly sits in the top of the AdBlue tank on a large sealed flange. That seal hardens with heat cycling and age. A weep here typically shows as crust on the top face of the tank and a drip that runs down the tank wall before it lands.
2. The pump or delivery module housing
Most systems put the pump, filter and pressure sensor into one module. The module body is plastic, and the internal seals are the usual failure point. A leak from here is often accompanied by pressure faults, because the same seal that is letting fluid out is letting pressure escape.
3. The dosing injector seal
The injector that sprays AdBlue into the exhaust is clamped into the pipe on a gasket, and it lives in the hottest part of the system. A failed injector seal leaves a distinctive crust ring on the exhaust pipe itself, often with staining running downwards from it.
4. Lines and quick-connectors
The heated lines between tank and injector use push-fit connectors. Road salt, stone chips and someone else’s jack in the wrong place all take their toll. A connector leak is frequently the cheapest fix on this list, and the one most often missed because the crust is hidden above a heat shield.
5. The filler neck
A crust of white deposit around the filler cap usually is not a leak at all — it is spillage from topping up, which dries in place. It is worth cleaning off with water rather than leaving it, because it can work its way into the neck and restrict filling.
6. The tank itself
Cracks in the tank body are less common but do happen, usually from an impact, from a badly supported jacking point, or from fluid freezing in a tank that was already stressed. Tank damage is the one case where a specialist AdBlue tank repair is the appropriate route rather than a seal or a connector swap.
Why the white crust is the diagnosis
The white deposit is doing you a favour. Liquid runs, drips and blows about at motorway speed, so a wet patch tells you very little about where it came from. Crystallised urea forms exactly where the fluid dried, which is usually within a few centimetres of where it escaped. Wash the underside, dry it, drive it, and look again: the first crust to reappear is your leak point.
It also creates a second problem. The same crystallisation that helps you find the leak will, inside the system, jam a level sender float, block a pump strainer or partially obstruct the dosing injector. We cover the internal version of this in detail in our guide to why AdBlue crystallises inside the system, and the practical removal steps in our walkthrough on clearing AdBlue crystals.
The point most guides miss: a leak and a crystallisation fault are not two separate problems. A leak vents fluid to air, air dries it to crystal, and crystal is what eventually triggers the fault code. Fixing the fault without fixing the leak buys you months, not years.
What happens if you leave it
Three things, in a fairly predictable order.
Corrosion of everything it touches. This is the part owners underestimate. The fluid manufacturer’s own materials guidance rules out zinc-coated steel, copper and copper alloys for contact with AdBlue. Underbody brackets, clips, earth straps and connector pins are frequently made of exactly those materials. A slow weep onto a wiring connector will do more expensive damage than the leak itself.
Consumption that does not add up. Long before a warning light appears, you may notice the tank emptying faster than it used to for the same mileage. A leak is the most common explanation for that, and it is worth checking before assuming the dosing strategy has changed.
A fault, then a countdown. Once the system cannot build or hold pressure, or the level reading stops making sense, the ECU logs a fault. Most manufacturers then begin a no-restart countdown. At that stage you are on a clock, and the escalation from a stored fault to a no-restart countdown is the point at which the repair stops being optional.
What a proper repair involves
A leak repair that lasts follows the same four steps whoever does it.
Clean and confirm. The underside is washed down with water — which is genuinely the correct way to deal with spilled AdBlue — dried, and the vehicle is run so the system pressurises. This separates an old historic stain from a live leak.
Pressure-test rather than guess. The system is commanded to run and the pressure reading watched. A module that will not hold pressure points somewhere different to one that holds pressure but still weeps.
Replace the failed part, not the assembly. A perished connector o-ring is a connector o-ring. Replacing an entire pump module because the crust was nearest to it is how repair bills get out of hand. Where the pump genuinely is at fault, our guide to AdBlue pump faults and the options available sets out what is repairable and what is not.
Clear the fault and verify dosing. The stored codes are cleared and the vehicle is driven so the system completes a dosing cycle and the monitors re-run. A cleared code on a stationary vehicle proves nothing.
Repair cost varies enormously depending on which of the six points is leaking — a connector seal and a cracked tank are not remotely the same job — so we diagnose first and quote after. Our mobile AdBlue diagnostics service is usually the sensible first step, because it is carried out at your home or yard rather than tying the vehicle up at a workshop.
Frequently asked questions
Is an AdBlue leak dangerous?
Not in the sense of fire or fumes — AdBlue is non-flammable and is not classed as a hazardous substance. The risk is corrosion to nearby metal components and wiring connectors, and the emissions fault that follows once the system can no longer dose correctly. It is a repair to book promptly rather than an emergency to pull over for.
Can I still drive with an AdBlue leak?
Usually yes, in the short term, provided no warning light or countdown has appeared. Keep the level topped up and get it looked at. Once a no-restart countdown starts, the clock runs down whether you drive the vehicle or leave it parked, so that is the point to stop waiting.
Will an AdBlue leak fail an MOT?
A leak is not itself a specified emissions test item, but the MOT inspection manual requires testers to reject a vehicle where visible emission control equipment is missing, obviously modified or obviously defective, and selective catalytic reduction components are named among the items to check. A visibly damaged tank or a disconnected dosing injector falls squarely into that. An illuminated emissions warning lamp is also assessed at test.
How do I clean up spilled AdBlue?
Plain water. AdBlue is water-soluble, so rinsing thoroughly is both the recommended method and the most effective one. Do it before the water evaporates and leaves crystal behind, and rinse any painted surface it has run down rather than wiping it dry.
Why is my AdBlue tank emptying so quickly?
A leak is the first thing to rule out, particularly if consumption has changed suddenly rather than gradually. Beyond that, a dosing injector stuck part-open, or a recent software change to the dosing strategy, will both raise consumption. A leak will normally leave physical evidence; the other two will not.
Found a puddle or white crust under your vehicle?
We diagnose AdBlue leaks at your home or business across the West Midlands and surrounding areas, using proper pressure testing rather than guesswork — so you pay to fix the part that has actually failed. Get in touch and tell us what you have found.
