Kenworth T680 Paccar MX-13 Derate - Active Code SPN 5394 FMI 5

Question

My 2018 T680 just hit a 5 MPH derate while hauling a full load through the mountains. I am getting a 'Low DEF Pressure' warning on the dash and the MIL is solid. I can hear the pump cycling but it doesn't seem to be building enough pressure to satisfy the ECM.

Jordan Lee · Accepted Answer

The issue was a cracked DEF pick-up tube inside the tank assembly (Part #1916301PE). It was sucking air whenever the tank dropped below half full, causing the SPN 5394 FMI 5 code. Replaced the tank sender unit, performed a forced SCR system prime and 30-minute stationary regen to clear the derate. Cost was about $850 for the unit plus 3 hours of labor.

Luis Hernandez · Answer

Look at the electrical connector at the DEF pump. These are prone to corrosion from road salt and can cause a high resistance fault that looks like a mechanical pressure issue.

Lina Brooks · Answer

Check the DEF dosing valve (injector) at the decomposition pipe. If it's salted up with urea, it won't atomize properly and the pressure sensor will read outside of parameters during the dosing cycle.

Rachel Nguyen · Answer

You need to run a DEF Pump Override test using DAVIE software. Watch the pressure ramp-up; if it struggles to reach 5 bar (approx 72.5 psi), your pump motor is likely failing internally or the pick-up screen in the tank is clogged.

Kenworth T680 Paccar MX-13 Derate - Active Code SPN 5394 FMI 5

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