AdBlue Crystallisation: Why It Happens and How to Fix It
White crystals around the AdBlue filler or under the car usually mean dried AdBlue. Sometimes it is harmless spillage. Sometimes it leads to blockages and repeat warnings. This guide shows you what it is, what it affects, and the quickest route to getting it sorted.
If you have spotted white crusty residue near the AdBlue filler cap, around pipe joints, or underneath the vehicle, you are probably looking at AdBlue crystallisation. It happens when AdBlue dries and leaves behind solid deposits.
In the early stages, it may just look messy. Over time, it can restrict flow, affect injector spray patterns, and contribute to warnings like “emissions fault”, “AdBlue quality”, and mileage countdowns. The good news is you can catch it early and avoid bigger headaches.
Typical signs you might have crystallisation
- • White residue around the filler neck or cap
- • White marks near AdBlue lines or connectors
- • A smell or damp patch near the tank area
- • AdBlue warnings that keep returning
- • A countdown message (start prevented / no restart)
If you are dealing with warnings at the same time, this guide helps you map message to next step:
what causes AdBlue warning lights and how to fix them
.
Why AdBlue crystallisation happens
AdBlue is a urea-based solution used in the SCR system. When it dries, the water content evaporates and leaves behind crystals. You see the crystals because they build up on surfaces and around joints.
The most common triggers
- Spillage during top-ups that dries on paintwork and plastics.
- Small leaks around seals, connectors, or lines.
- Weak dosing patterns where spray does not atomise properly and residue builds up.
- Cold weather use where marginal parts struggle and deposits worsen.
- Repeated short trips where the system does not run in stable conditions for long enough.
If your issues started in winter or get worse when temperatures drop, this is a useful read:
how to avoid AdBlue-related breakdowns this winter
.
What crystallisation can affect in the system
Crystals do not just sit there. If they build up in the wrong place, they can restrict flow and change how the system behaves. That is when warning lights start appearing more often.
Filler neck and cap area
Usually from spillage. It looks dramatic but is often the easiest to deal with if caught early.
Lines and connectors
Small leaks can dry and form deposits. Over time, this can cause restrictions and trigger pressure or dosing faults.
Injector area
Deposits can affect spray patterns. When dosing becomes inconsistent, the ECU can flag performance and quality issues.
Pump and pressure control
Restrictions and build-up can make the pump work harder. This can contribute to repeat faults if the system cannot build pressure reliably.
If you suspect pump or pressure issues are part of the problem, this guide explains the common fault patterns:
AdBlue pump repair explained
.
What you can do safely (and what to avoid)
The goal is to stop the cause and avoid making it worse. If you only have residue around the filler cap, that is often manageable. If you have repeat warnings or a countdown, it is time for diagnostics.
Safe steps
- Wipe up fresh spills straight away after topping up.
- Use a proper nozzle or funnel to reduce spillage.
- Use sealed, in-date AdBlue from a known supplier.
- Take a photo of where the crystals are and when they appeared.
Avoid these
- Do not keep topping up to “force” a reset.
- Do not keep clearing warnings if they return.
- Do not ignore a countdown message.
- Do not assume crystals mean only one part has failed.
How crystallisation gets fixed properly
The best fix depends on where the deposits are and whether the vehicle is already failing system checks. Cleaning visible residue is only one part. If there is restriction, leakage, or dosing issues, the system needs a proper assessment.
What a proper visit focuses on
- Confirm the fault path using diagnostic codes and live data.
- Inspect lines and joints for leaks, residue, and restrictions.
- Assess dosing behaviour so the fix targets the real cause, not a guess.
- Confirm the result so warnings clear for the right reason.
If you are already in a countdown, this guide explains what to do next:
AdBlue start prevented message
.
Crystals showing up or warnings keep returning?
Send a quick photo of the residue and your dash message. We will advise the best route and book a mobile visit across the West Midlands if needed.
Serving West Midlands and surrounding areas.
Helpful links
AdBlue warning lights
What warnings mean and the right next step.
AdBlue removal
A long-term route when the system keeps failing checks.
Contact us
Send your reg and message for quick advice.
