2018 Freightliner Cascadia - SPN 3221 FMI 5 NOx Sensor Issues

Question

My DD15 is throwing a SPN 3221 FMI 5 code for the Aftertreatment 1 Intake NOx Sensor. The check engine light is on and I am worried about the unit going into a 5 mph derate while I am under a heavy load. It seems to stay active even after clearing the codes with my scan tool.

Pete Sanders · Accepted Answer

The issue was a failed internal heating element in the Inlet NOx sensor itself. I replaced the sensor with a genuine Detroit part (A0101532228). After installation, I had to perform a SCR Efficiency reset using DiagnosticLink 8.16 to clear the permanent fault. The sensor cost $540 and it took about 45 minutes to install once the rusted threads were soaked in PB Blaster. Torque the sensor to 50 Nm to ensure a good seal.

Carlos Diaz · Answer

FMI 5 usually points to an open circuit. Before you buy a sensor, check the Power Distribution Center fuses; sometimes a blown fuse for the aftertreatment system can trigger multiple sensor communication codes.

Ashley Park · Answer

Use a multimeter to check for 120 ohms across the CAN pins at the sensor connector with the batteries disconnected. If you see high resistance or an open, the problem is definitely in your chassis harness and not the sensor itself.

Dave Kowalski · Answer

Check the wiring harness near the back of the engine where it drops down to the frame rail. These Cascadias are notorious for the harness rubbing against the bracket, which can chafe the CAN HI/LO wires and cause intermittent NOx sensor faults.

2018 Freightliner Cascadia - SPN 3221 FMI 5 NOx Sensor Issues

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