Cummins Fault Code

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SPN 3216 FMI 20

Aftertreatment Derate

Variant 1

System

Aftertreatment

Symptom

Aftertreatment Derate

Diagnosis & fix steps

LOGIC: Inlet NOx Sensor Drifted High. ACTION: Perform an "Analyzer" test on the NOx sensor. Replace if reading >50ppm at idle.

Red Seal field insights

Logic: The sensor is "talking" but giving inaccurate data. FIX: This often triggers a "False" DPF soot load warning.

Variant 2

System

Not specified

Symptom

DEF usage is high or low; Frequent parked regens; "Derate" timer may start; Check engine light.

Diagnosis & fix steps

DEF usage is high or low; Frequent parked regens; "Derate" timer may start; Check engine light. LOGIC: NOx sensor reading is "drifting" out of range compared to the outlet sensor. ACTION: Check for exhaust leaks upstream of the sensor. Inspect sensor tip for soot/moisture contamination. FIX: Repair exhaust leaks or replace the Inlet NOx Sensor (and its control module).

Red Seal field insights

NOx sensors are extremely sensitive to moisture. Never "wash" an exhaust system with a pressure washer while the sensors are hot, or you'll crack the ceramic element inside.

Same code keeps coming back?

That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.

Book an uptime review

Read before you touch anything

  • This is general reference information, not a repair manual. It is for trained, qualified technicians.
  • Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
  • Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
  • You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
  • Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.

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