AdBlue additives: what affects the legal warranty, and what affects the SCR system

Anti-crystallisation additives for AdBlue are sold freely and are sometimes presented as a solution to SCR problems. The legal question is often stated too broadly: such an additive does not cancel the legal guarantees owed to a car owner. The useful rule is more precise, and it concerns dosage, the manufacturer’s optional commercial warranty, and the status of any current claim.

The reference standard

AdBlue is defined by ISO 22241: 32.5% urea and 67.5% demineralised water. At the prescribed dose — around 10 ml per 10 litres, or 0.1% — the liquid remains within that standard. Pre-additivated AdBlue is also sold in drums and cubitainers. The problem begins with overdosage: two to three times the dose changes the composition and can clog the SCR system with deposits.

What legal guarantees cover

In France, the legal guarantee of conformity (articles L217-1 et seq. of the Consumer Code) is owed by the seller and is a matter of public order. Any clause that excludes it is deemed unwritten. No additive can remove it. The hidden-defects guarantee (article 1641 of the Civil Code) requires a defect that existed before the sale; a product poured in later does not remove that guarantee.

The manufacturer’s commercial warranty is different. It is a voluntary contract and may exclude a non-approved product, but it cannot reduce the legal guarantees. This distinction matters when a dealer or manufacturer points to an additive in a refusal to cover a repair.

The practical risk

What an additive actually does is provide the manufacturer with a causality argument for refusing coverage. That argument can be discussed. It is not the same as a legal cancellation of warranty rights. Where a case is in progress against the manufacturer, the prudent course is to abstain from adding a product until the position is clear. Outside the commercial warranty and without an active claim, there is no legal risk in the additive itself. In all cases, the dosage must be respected.

Rule to remember

• Do not treat anti-crystallisation additives as automatically prohibited.
• Do not confuse the optional commercial warranty with the legal guarantees.
• Respect the prescribed dose; overdose is the documented risk to the SCR system.
• If a manufacturer refuses coverage because of an additive, seek advice on consumer protection and hidden-defect law.

Owners can document their case through our registry. If your vehicle has an AdBlue or Haldex® failure, you can register it at defect-registry.com to help map these defects.