Diesel exhaust fluid, or DEF, is part of the SCR emissions system on modern diesels, including later Power Stroke, Cummins, and Duramax trucks. It is simple to use, but quality and storage matter more than many owners realize.
What DEF does
DEF is a precise mix of urea and purified water that is injected into the exhaust, where it helps convert harmful nitrogen oxides into harmless nitrogen and water. The truck meters it automatically; your job is to keep the tank filled with good fluid.
Why quality and storage matter
DEF has a shelf life and degrades with heat and time. Old or contaminated fluid, or the wrong fluid entirely, can trigger quality faults and even a power derate. Store DEF in a cool place, keep it sealed, and use it within its shelf life. Never dilute it or use anything other than proper DEF.
Avoiding DEF-related derates
- Keep the DEF tank from running empty, which forces a derate
- Use fresh, properly stored fluid from a sealed container
- Address DEF quality and injector faults promptly
- Do not let DEF freeze in the container long-term or overheat in storage
Handling tips
DEF crystallizes when it dries, so wipe up spills and keep the fill area clean. Use a dedicated spout or container to avoid contamination. Treated as the simple consumable it is, DEF keeps the emissions system happy and the truck out of derate.
Frequently asked questions
What happens if I run out of DEF?
The truck will warn you and then force a power derate to protect the emissions system. Keep the tank from running empty.
Does DEF go bad?
Yes. It degrades with heat and time. Store it cool and sealed and use it within its shelf life to avoid quality faults.
Can I use anything other than DEF?
No. Use only proper diesel exhaust fluid. Diluting or substituting causes faults and can trigger a derate.
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