Every BS-VI diesel truck, bus and tractor built since April 2020 carries a second tank alongside the fuel tank — usually with a blue filler cap. That's for Diesel Exhaust Fluid (DEF), also known by the trademarked name AdBlue® or the generic term AUS 32. Here's what it does and why the vehicle won't run properly without it.

What DEF actually is

DEF is a precisely mixed solution of 32.5% high-purity urea in demineralised water — not a fuel additive, and not mixed with diesel at any point. It's stored in its own tank and injected directly into the exhaust stream, upstream of a Selective Catalytic Reduction (SCR) catalyst.

How it cuts emissions

Inside the hot exhaust stream, DEF decomposes into ammonia, which reacts with the SCR catalyst to convert harmful nitrogen oxides (NOx) into harmless nitrogen gas and water vapour — the same nitrogen and water that make up most of the air we breathe. This is what allows modern diesel engines to meet BS-VI and Euro IV–VI NOx limits without sacrificing fuel efficiency the way some earlier emissions strategies did.

Why purity matters

DEF must be manufactured to ISO 22241-1 / IS 17042 (Part 1) tolerances — the urea concentration, conductivity and trace-metal content are tightly controlled. Off-spec DEF (wrong concentration, contaminated, or diluted) can damage the SCR catalyst or trigger a fault code that limits engine power (a "derate") until corrected. This is also a BIS-regulated category requiring a valid ISI licence to sell — always confirm current certification status with your supplier.

Handling tips

  • Never add DEF to the fuel tank, or diesel to the DEF tank — cross-contamination damages the SCR system.
  • Store between 5°C and 25°C where possible; DEF crystallises below −11°C (fully recoverable on thawing) and slowly degrades above 30°C.
  • Use only DEF-dedicated containers, pumps and fittings (stainless steel, HDPE or DEF-rated polymer) — ordinary metal fittings can corrode over time.

AGREX DEF is manufactured to ISO 22241-1 / IS 17042 (Part 1) at our GIDC Matar facility, available from 5 L containers through bulk tanker. See full specifications on our AGREX DEF product page.