2018 Freightliner Cascadia DD15 SPN 3216 FMI 20 - SCR Efficiency Low

Question

I am getting a dash warning for low SCR efficiency and the truck is threatening a 5 mph derate in 4 hours. The NOx sensor readings look a bit erratic on my laptop but I can't tell if it's the sensor or the DEF quality causing the issue. This started right after a long uphill pull through the mountains last night.

Rachel Nguyen · Accepted Answer

The issue was a heavily crystallized DEF Dosier Valve (Part# A0001402039). I removed the injector and found a massive 'elephant foot' of urea buildup blocking the spray pattern into the decomposition pipe. I cleaned the pipe, installed a new genuine Bosch injector, and performed an SCR Efficiency High Load Service Routine using DiagnosticLink 8.15. Total cost was about $450 for the part and two hours of labor, and the derate cleared immediately once the NOx conversion stayed above 80% during the test.

Jordan Lee · Answer

Hook up DiagnosticLink and perform a DEF Quantity Test. You should be seeing approximately 100ml to 120ml in a graduated cylinder over a standard test cycle. If it's low, replace the pump filter.

Lina Brooks · Answer

You need to check the DEF doser nozzle first. If it is crystallized or plugged, you won't get enough urea into the stream to satisfy the downstream NOx sensor, which triggers the efficiency code.

wrenchqueen · Answer

Check your inlet and outlet NOx sensor readings during a steady cruise. If the outlet is higher than the inlet, or if they are nearly identical after the catalyst is up to temp, your SCR brick might be face-plugged or the sensor is dead.

2018 Freightliner Cascadia DD15 SPN 3216 FMI 20 - SCR Efficiency Low

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