AFTERTREATMENT SUBSYSTEMe.g. SPN 4364 FMI 18
Aftertreatment Diagnostic Guide
Full aftertreatment system diagnostics — DPF, SCR, DOC, DEF, and NOx sensor as an integrated system.
Common Symptoms
- 1Multiple aftertreatment fault codes active
- 2Derate or shutdown active
- 3Regen inhibited or failed
- 4NOx efficiency below threshold
- 5DEF quality or level fault
- 6Aftertreatment temperature out of range
Root Causes (Ranked by Frequency)
- Cascading faults from single upstream cause
- DOC degradation affecting DPF regen temperature
- DEF system failure affecting SCR efficiency
- Exhaust temperature sensor failure
- Aftertreatment injector failure
- Multiple sensor failures from wiring issue
Diagnostic Process — Step Preview
Structured diagnostic sequence — evidence-gated at each step
4 steps locked behind Technician Mode- 1INTAKE → full aftertreatment fault map
- 2SENSOR_VALIDATION → all AFT sensors
- 3FLOW_PRESSURE_CHECKS → exhaust flow and temp
- 4PLAUSIBILITY_CHECKS → cascade fault analysis
- 5VERIFICATION → full system efficiency check
Unlock the full diagnostic sequence, DVOM tests, and repair procedures in Technician Mode.
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DOWNSTREAM EFFECTfix upstream first
Fuel delivery low → aftertreatment temperature anomaly
Low fuel delivery causes misfires and unburned fuel entering aftertreatment, causing temperature spikes.
Low fuel rail pressure → injector misfires
Misfires → unburned fuel in exhaust
Unburned fuel → aftertreatment temperature spike
Temperature spike → DPF/SCR damage risk
Ready to diagnose?
The Aftertreatment diagnostic engine walks you through a structured test sequence — no guesswork.
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