What is DEF fluid? It’s diesel exhaust fluid, a clear mix of 32.5% urea and 67.5% deionized water that cleans up diesel exhaust.
Your truck sprays DEF into the exhaust, where the SCR system turns harmful NOx gas into nitrogen and water vapour. One Canadian parts retailer says SCR can cut NOx by up to 90%.
DEF has its own tank, usually with a blue cap. It never goes in the fuel tank. If it does, don’t start the engine. One US fuel supplier says starting it pushes the bad fuel through the system, so tow it to a shop to drain the tank.
Here’s what to know:
- Freezing: DEF freezes at -11°C, but most tanks have a heater to thaw it.
- Storage: keep it in a cool, dry spot out of direct sunlight.
- Running low: a warning light comes on first.
- Running out: the engine loses power and speed, and some trucks won’t start.
Keep the tank topped up, and never add anything to DEF. Our Surrey and Port Moody shops carry heavy duty parts. For the full list, read our heavy duty truck maintenance checklist.
Service desk The full guide
Heavy duty truck maintenance checklist: the full checklist, step by step.