Universal HD Fault Code
Free · no signupSPN 4364 FMI 18
DEF Lamp Solid / 25% Engine Torque Derate
System
Aftertreatment / SCR
Symptom
DEF Lamp Solid / 25% Engine Torque Derate
Diagnosis & fix steps
LOGIC: Catalyst System NOx Conversion Efficiency Below Acceptable Limit.
ACTION: Test DEF fluid quality using an optical refractometer (spec: 32.5% ± 0.7%). Remove DEF dosing injector to inspect for white crystallized urea clogs in the exhaust boss. Check live data ppm readings between upstream engine-out and downstream tailpipe NOx sensors.
Red Seal field insights
Sensor Drift vs. Fluid Contamination: The ECM continuously divides downstream NOx by upstream NOx to verify chemical conversion efficiency across the catalyst. Contaminated fluid, bad urea atomization, or drifted sensor probe ceramics will flag poor conversion.
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.
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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