SPN 3226 FMI 2 — Aftertreatment Outlet NOx Sensor Data Erratic
Erratic downstream NOx is diagnosed by paired sensor monitoring, heater checks, and leak inspection before replacement.
Fault Overview
SPN 3226 is the Aftertreatment Outlet (downstream) NOx sensor. FMI 2 indicates erratic or intermittent signal behavior.
Engine & Platform Coverage
Symptoms
- •Outlet NOx erratic
- •Downstream NOx sensor intermittent
- •SPN 3226 FMI 2
- •SCR outlet NOx unstable
Possible Causes
- 1.Intermittent connector/wiring
- 2.Failing sensor
- 3.Heater intermittents
- 4.Exhaust leaks at or after the SCR
- 5.Contamination or water intrusion
- 6.Ammonia slip creating unstable readings on some sensor designs
Wiring & Circuit Notes
Wiggle-test connector and harness. Inspect for heat and corrosion damage downstream of the SCR.
DVOM Specs & Sensor Tests
Downstream NOx should be lower and relatively stable when the SCR is functioning. Heater status must be solid. Rapid swings with stable upstream NOx point to the downstream sensor or its circuit.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Step 1 — Monitor Upstream and Downstream NOx Together Under Steady Conditions: Monitor upstream and downstream NOx together under steady conditions. Downstream NOx should be lower and relatively stable when the SCR is functioning.
- 2Step 2 — Check Downstream Heater Status and Current: Check downstream heater status and current. Heater must be solid for accurate readings.
- 3Step 3 — Wiggle-Test the Connector and Harness: Wiggle-test the connector and harness while monitoring live data. Downstream sensors fail from thermal shock and contamination.
- 4Step 4 — Inspect for Exhaust Leaks at the SCR Outlet and Beyond: Inspect for exhaust leaks at the SCR outlet and beyond. Exhaust leaks after the SCR also produce erratic oxygen dilution.
- 5Step 5 — Verify Power, Ground, and Communication: Verify power, ground, and communication circuits.
- 6Step 6 — Replace Sensor Only After Proof: Replace sensor only after electrical and leak proof. Rapid swings with stable upstream NOx point to the downstream sensor or its circuit.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Repair connector/harness $100–300; repair leak $100–400; replace sensor $300–900.
Start Diagnosing This Fault Now
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Summary
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