Universal HD Fault Code

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SPN 3226 FMI 04

Yellow Emissions Lamp / SCR Derate Imminent

System

Sensors / Exhaust

Symptom

Yellow Emissions Lamp / SCR Derate Imminent

Diagnosis & fix steps

LOGIC: Outlet NOx Sensor Circuit Voltage Below Normal (Short to Ground). ACTION: Disconnect downstream tailpipe NOx sensor harness. Measure sensor supply voltage (12V/24V) and verify sensor ground. Check sensor signal harness where it runs near the tailpipe bracket for heat-melted insulation shorted to the frame rail.

Red Seal field insights

Thermal Degradation: Downstream NOx sensor pigtails operate under extreme heat cycling and vibration. Melting loom shorts the internal power or signal wires to the chassis ground, disabling tailpipe emissions monitoring.

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.

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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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