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

Question

Running down the interstate and got a check engine light followed by a '5 MPH derate in 60 minutes' warning. The truck seems to be pulling fine but the dash says the SCR is inefficient. I already topped off the DEF tank from a fresh jug but the light won't go away.

hd_trucker_77 · Accepted Answer

The issue was a completely clogged DEF Metering Unit nozzle (A0001405339). I pulled the injector and it was covered in white urea crystals. I cleaned the port, installed a new genuine Bosch doser valve, and performed the 'SCR Efficiency Component Verification' specialized test in DiagnosticLink to reset the monitors and clear the derate. Total part cost was about $420 plus two hours of labor.

Luis Hernandez · Answer

Check your inlet and outlet NOx sensors. If the sensors are biased, the ACM will think the SCR isn't doing its job. Usually, a failed NOx sensor will throw its own code, but not always.

Sarah Thompson · Answer

You need to check the DEF doser nozzle first. They are notorious for crystalizing and blocking the spray pattern, which causes the SCR catalyst to under-perform because it isn't getting enough urea.

backyard_bart · Answer

Since you did a parked regen and it didn't clear, run a DEF quantity test using DiagnosticLink. You should see about 100ml to 120ml in a 60-second test. Low volume usually means a failing pump or a leak in the line.

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

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