# What Is DEF and Can You Accidentally Misfuel It?

> DEF is becoming standard on modern diesel vehicles in New Zealand. Putting diesel in the DEF tank — or DEF in the fuel tank — is a costly mistake. Here is what drivers need to know.

- Canonical URL: https://www.eekmechanical.com/news/def-misfuel-what-is-def
- Published: 2024-06-03
- Author: EEK Mechanical
- Publisher: EEK Mechanical, a trade name of Forman Pacific, LLC

## What Is DEF?

DEF is a diesel exhaust fluid — a solution of urea and deionised water — used in Selective Catalytic Reduction (SCR) systems to reduce nitrogen oxide emissions from diesel engines. It is now fitted as standard on most Euro 6-compliant diesel vehicles sold in New Zealand since 2017, including popular models from Toyota, Volkswagen, Mercedes-Benz, and Ford.

## How DEF Misfuelling Happens

There are two common DEF misfuel scenarios. The first is filling the DEF tank (typically a separate, smaller filler cap near the main fuel filler or in the boot) with diesel or even water. The second — and far more damaging — is pouring DEF into the main diesel fuel tank. DEF is highly corrosive to fuel system components and will damage injectors, the high-pressure pump, and fuel lines rapidly. Even a small quantity of DEF in a diesel tank requires immediate manufacturer-spec fuel system recovery.

## What to Do

If you suspect an DEF misfuel — in either direction — do not start the engine. An SCR system exposed to diesel will crystallise and block the catalyst; a fuel system exposed to DEF will begin corroding within minutes of running. Call EEK Mechanical on [(866) 777-3712](tel:8667773712). Our technicians are trained to handle DEF contamination scenarios and can advise on whether the SCR system or fuel system is affected.

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Source: https://www.eekmechanical.com/news/def-misfuel-what-is-def