Short answer
Diesels with selective catalytic reduction (SCR) inject diesel exhaust fluid (DEF) into the exhaust to cut NOx. If the system sees low or poor-quality DEF, a dosing problem, or a sensor fault, it warns you and can limit power or speed. DEF that loses water, gets contaminated or is stored hot can leave white urea crystals that block the doser.
Start with the easy checks
- Check the DEF level and top up with sealed, certified DEF from a clean container.
- Never put anything but DEF in the DEF tank, and never use funnels or jugs that have held fuel or oil.
- Store DEF out of direct sun and heat; heat shortens its shelf life.
- Look (engine cool) for white crust around the doser and lines.
- Read codes: level, quality, dosing and NOx sensor faults point to different fixes.
Likely causes, most common first
- Low or poor-quality DEF
- Old, overheated or contaminated DEF can trigger quality warnings.
- Crystallization
- When water evaporates or DEF is off-spec, urea deposits can form on the doser tip, lines, mixer and catalyst.
- Doser, pump or heater faults
- Hardware in the DEF system can fail.
- NOx sensor faults
- See the NOx sensor page.
These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.
When to stop and see a diesel tech
- Derate or countdown warnings that do not clear after a DEF refill.
- Deposits in the exhaust or SCR.
- Fuel or other fluid added to the DEF tank: do not start the engine; get it towed.
Questions
Does DEF go bad?
Yes. Industry guidance says heat shortens DEF shelf life and contamination ruins it. Buy sealed product and store it cool and clean.
What causes DEF crystals?
Water loss, off-spec or contaminated DEF, and dosing problems can leave urea deposits on the doser and in the exhaust.
Related symptoms
How do I know if my diesel NOx sensor is bad?
Troubleshoot →Power & drivabilityWhy did my diesel lose power or go into limp mode?
Troubleshoot →EmissionsWhat is DPF regeneration and why does my truck keep doing it?
Troubleshoot →Where this comes from
- Cummins: DEF quality and maintenance (PDF)
- API: DEF do's and don'ts fact sheet (PDF)
- Fluid Life: Common DEF problems and how to test for them
Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.