Top failure modes
Contaminated DEF (water, diesel, or off-spec fluid), crystallized injector, failed quality sensor, leaking dosing line, plugged supply pump.
DEF problems escalate fast — from warning lamp to derate to no-start in three stages. Here's the diagnostic order.
Most DEF derates are caused by contaminated fluid or a leaking dosing module, not the SCR catalyst itself.
Contaminated DEF (water, diesel, or off-spec fluid), crystallized injector, failed quality sensor, leaking dosing line, plugged supply pump.
1) Test DEF quality with a refractometer (32.5% urea ± 0.7%). 2) Inspect dosing injector + heater. 3) Check dosing supply pump pressure. 4) Test quality sensor. 5) Last — SCR catalyst.
Use API-certified DEF only, don't refill from open totes left in summer sun, replace DEF filter at intervals (often missed), inspect heater operation at every PM.
Yes — DEF degrades over time, especially above 86°F. Sealed shelf life is 1–2 years; in-vehicle life is 6–12 months.