SPN 3226 FMI 4 — Aftertreatment Outlet NOx Sensor Circuit Voltage Below Normal
Outlet NOx sensor low-circuit codes require short isolation before sensor replacement. Repeat outlet sensor failures from unresolved shorted looms after AT service are the most documented pattern.
Fault Overview
SPN 3226 FMI 4 indicates the aftertreatment outlet NOx sensor circuit voltage is below the normal minimum — a short circuit or low-signal class fault. Harness short, failed sensor module with internal short, water intrusion, and crushed AT loom are the primary causes. Repeat outlet sensor failures from unresolved shorted looms is the most documented pattern.
Engine & Platform Coverage
Possible Causes
- 1.Harness short to ground at clamp edges on aftertreatment box and frame-rail transitions
- 2.Failed outlet NOx sensor module with internal short
- 3.Water intrusion at connector seals — moisture bridging to ground
- 4.Crushed AT loom from improper routing during service
- 5.Melted insulation near heat sources downstream of DPF/SCR
Wiring & Circuit Notes
Inspect clamp edges on the aftertreatment box and frame-rail transitions — these are the highest-probability short locations for the outlet NOx circuit. Check for melted insulation near heat sources downstream of the DPF and SCR. Inspect connector seals for moisture intrusion.
DVOM Specs & Sensor Tests
Disconnect the sensor module and check for shorted power and signal lines in the harness. Inspect connector seals for moisture intrusion. Replace the sensor only if the circuit is clean — installing a new sensor into a shorted harness will damage the replacement.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Disconnect outlet sensor module connector
- 2Check harness for short to ground on power and signal lines
- 3Inspect connector seals for moisture intrusion
- 4Inspect harness at AT box clamp edges and frame-rail transitions for melted insulation
- 5Repair harness short or seal connector before sensor replacement
- 6Replace sensor module only after circuit is confirmed clean
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Disconnect outlet sensor module connector
- 2Check harness for short to ground on power and signal lines
- 3Inspect connector seals for moisture intrusion
- 4Inspect harness at AT box clamp edges and frame-rail transitions for melted insulation
- 5Repair harness short or seal connector before sensor replacement
- 6Replace sensor module only after circuit is confirmed clean
Unlock full diagnostic steps, DVOM tests, and repair procedures.
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Summary
SPN 3226 FMI 4 requires short isolation and connector seal inspection before outlet sensor replacement. An unresolved harness short will damage replacement sensors.