Reductant Injector

📖 YOUCANIC Automotive Glossary

The reductant injector (also called the DEF injector, urea injector, or AdBlue injector) is the electronically controlled nozzle that sprays a precisely metered amount of AdBlue/DEF (urea solution) into the exhaust stream upstream of the SCR catalyst. The ECU controls injection timing, duration, and quantity based on upstream NOx sensor readings, exhaust temperature, exhaust flow rate, and SCR catalyst temperature to achieve optimal NOx conversion efficiency. The injector operates in a high-temperature, high-vibration environment and is typically cooled by DEF fluid circulation or dedicated coolant lines to prevent heat damage and crystallization.

Reductant injector problems are a common cause of SCR system failures and aftertreatment DTCs. Crystallization (white deposits forming at the injector tip from DEF evaporating before fully mixing with exhaust) is the most frequent issue, caused by low exhaust temperatures, injector spray pattern degradation, or mounting angle issues. Symptoms include SCR efficiency codes, NOx threshold exceeded warnings, DEF consumption changes, and progressive derate. The YOUCANIC UCAN-II can display DEF injection rate in live data and may perform injector cleaning or test cycles on some vehicles. After injector replacement, system resets through the scanner are typically required.