One of the most common issues with the Def Doser is that it goes bad and starts leaking. And most of the time, you get a code for SCR Efficiency because instead of spraying, it's leaking so it doesn't reduce NOx. One of the possible fault codes could be 3568, 5394.
Find the place where Def Doser is installed. Then disconnect the electrical connector and unplug the DEF line (1). After that, clamp up engine coolant hoses to avoid coolant drain off. Unplug the coolant hoses (2).
Some engine coolant will drain off, so prepare a designated coolant container to hold the coolant while you continue the repair.
Unscrew the bolts and remove the old DEF Doser. Clean up the Dosing Unit mounting surface on the vehicle and around the hole on the exhaust pipe from urea crystals. Then replace the gasket if needed and install the new Def Doser. Reconnect all hoses and electrical connectors. Then unclamp coolant hoses, start the engine and check for leakage. If needed, calibrate the Dosing Module with diagnostic equipment.
Have a good ride!