When the DEF injector malfunctions, the impact extends beyond the exhaust system to affect the normal operation of the engine itself. Specifically, the following symptoms and consequences may arise:

I. Core Emission System Failure

Nozzle Clogging due to Urea Crystallization:Urea within the DEF (Diesel Exhaust Fluid) can crystallize, harden, and accumulate at the injector nozzle tip. This blockage prevents the precise injection of the urea solution into the exhaust system, ultimately causing tailpipe nitrogen oxide (NOx) emissions to exceed regulatory limits and directly triggering the illumination of the malfunction indicator lamp.
**Exhaust Aftertreatment System Failure:** The SCR (Selective Catalytic Reduction) process cannot be completed. Consequently, the vehicle's OBD system will identify the emissions as non-compliant, preventing the vehicle from passing its annual inspection; in some cases, this will also trigger specific emission-related diagnostic trouble codes.
II. Engine Operation Anomalies

Power Limitation and "Limp-Home" Mode: To enforce operational restrictions, the vehicle's ECU activates a protective mechanism that cuts off a portion of the power output. Vehicle speed is restricted to a low range, making normal acceleration or high-speed driving impossible. For vehicles equipped with common-rail fuel systems (specifically US17 and newer engine models), severe malfunctions may result in a complete failure to start

Fuel System Abnormalities: If DEF infiltrates the fuel system, it can corrode the internal components of the fuel injectors. Furthermore, the crystallization of urea can clog fuel lines, leading to engine misfires (characterized by noticeable engine shaking during idling or driving) and causing abnormal fuel pressure fluctuations within both the high- and low-pressure fuel circuits, which are subsequently recorded as diagnostic trouble codes.

Electrical Faults and Warning Alarms:Corrosion and crystallization can compromise the injector's electrical circuitry. This triggers the recording of various electrical fault codes—such as short circuits, open circuits, or voltage anomalies—and directly illuminates the engine malfunction indicator lamp.