Cummins Fault Code
Free · no signupSPN 3521 FMI 2
Derate / Check Engine Light
Variant 1
System
Emissions
Symptom
Derate / Check Engine Light
Diagnosis & fix steps
LOGIC: SCR Catalyst Conversion Efficiency Erratic/Incorrect.
ACTION: Inspect DEF doser injector for crystallization/clogging. Sample DEF quality with a refractometer (must read 32.5% concentration).
Red Seal field insights
Operation Fault: Triggered when bad batch DEF or improper storage degrades fluid concentration, causing inaccurate conversion readings.
Variant 2
System
Emissions
Symptom
Engine Derate / Check Engine Light
Diagnosis & fix steps
LOGIC: SCR Catalyst Conversion Efficiency Erratic/Incorrect.
ACTION: Inspect DEF doser injector for crystallization/clogging. Sample DEF quality with a refractometer (must read 32.5% concentration).
Red Seal field insights
Operation Fault: Triggered when bad batch DEF or improper storage degrades fluid concentration, causing inaccurate conversion readings.
Track this across your whole fleet
This code is free to read. AM Fleet Integrity lets your team log fault codes against assets, plus the full fault-code library, QR operator reporting, PM scheduling and predictive alerts. Start free — no credit card required.
Start your free trial