Kenworth T680 SPN 3221 FMI 5 - NOx Sensor Open Circuit

Question

My 2018 T680 just threw a check engine light and the dash is showing an SPN 3221 FMI 5 code. I noticed my DEF consumption has dropped off lately but the truck isn't in derate yet. The fault seems to stay active regardless of whether the engine is hot or cold.

Rachel Nguyen · Accepted Answer

Found the issue was a broken power wire inside the loom about 6 inches back from the sensor plug. I replaced the Aftertreatment 1 Intake NOx sensor (Part #22790-5M00A) because the internal heater element had also failed during testing. After installing the new sensor and clearing the codes with DAVIE, the truck completed a parked regen and the fault stayed inactive. Cost was about $650 for the PACCAR genuine sensor.

Sam Holloway · Answer

These MX engines are notorious for the aftertreatment harness getting brittle. Make sure you don't have a pin pushed back in the actual sensor pigtail side, as that can trigger the open circuit fault intermittently.

Megan Rivera · Answer

FMI 5 usually points to an open circuit. You should check the wiring harness between the sensor and the EECU, specifically looking for any rub spots near the frame rail where the loom might have slumped.

Brian O'Connor · Answer

Before you buy a sensor, check the voltage at the connector. You should have 12V or 24V constant power, a solid ground, and about 2.5V on the CAN High/Low lines. If power is missing, it's a harness issue.

Kenworth T680 SPN 3221 FMI 5 - NOx Sensor Open Circuit

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