2018 Freightliner Cascadia DD15 SPN 5246 FMI 15 - Derate and SCR efficiency low

Question

My truck is stuck in a 5 mph derate after several unsuccessful park regens. It shows SCR efficiency low and the DEF light is flashing even though the tank is full. I need to get this moving because I am sitting at a rest area with a hot load.

Jordan Lee · Accepted Answer

Managed to solve this by replacing the DEF Doser Valve (A0001405339) and the outlet NOx sensor. The doser was leaking internally, causing crystallized 'rocks' to block the decomposition pipe, which I had to chip out manually. After replacing the parts, I performed a forced DPF regen followed by an SCR Efficiency Test in DiagnosticLink to clear the 5 mph derate. Total parts cost was around $1,150 but it pulled the truck out of fault mode immediately.

Mike Reynolds · Answer

Check for white crystalline buildup inside the decomposition tube. If there is a massive build-up of urea, the DEF cannot mix properly with the exhaust gas to neutralize the NOx.

obd2_owl · Answer

You need to perform a DEF quantity test. Even if the nozzle looks clean, the pump might not be providing enough pressure to the injector, or the air supply to the nozzle is restricted.

torque_terry · Answer

Check your inlet and outlet NOx sensors using DiagnosticLink. If the values are too close together after the SCR warms up, the computer thinks the catalyst is dead or the DEF is diluted.

2018 Freightliner Cascadia DD15 SPN 5246 FMI 15 - Derate and SCR efficiency low

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