SPN 3226 FMI 3 — Aftertreatment Outlet NOx Sensor Circuit Voltage Above Normal
Outlet NOx sensor high-circuit conditions are electrical supply and communication diagnostics first. Both NOx sensors replaced when only the outlet feed was open is the most documented misdiagnosis — the upstream sensor was functioning correctly and did not need replacement.
Fault Overview
SPN 3226 FMI 3 indicates the aftertreatment outlet (downstream) NOx sensor circuit has a high voltage or open condition. Open power or ground feed, unplugged sensor module, failed sensor module, and heat-damaged harness are the primary causes. Both NOx sensors replaced when only the outlet feed was open is the most documented misdiagnosis.
Engine & Platform Coverage
Possible Causes
- 1.Open power feed to outlet NOx sensor module
- 2.Open ground feed to outlet NOx sensor module
- 3.Unplugged outlet sensor connector after AT service
- 4.Failed outlet NOx sensor module with internal open
- 5.Heat-damaged harness at outlet sensor location — downstream of DPF/SCR
Wiring & Circuit Notes
Verify power and ground at the outlet sensor module connector independently from the upstream sensor. Compare the presence of data from the outlet sensor versus the upstream sensor — if upstream data is present and outlet data is absent, the fault is isolated to the outlet circuit. Inspect AT module harness routing at the outlet sensor location.
DVOM Specs & Sensor Tests
Verify supply voltage and ground at the outlet sensor module connector. Verify communication signal where applicable. Compare outlet sensor data presence against upstream sensor data — a functioning upstream sensor with absent outlet data confirms the fault is in the outlet circuit, not a system-wide supply issue. Replace only the outlet sensor after its circuit is proven.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Compare outlet sensor data presence against upstream sensor data
- 2Verify power and ground at outlet sensor module connector
- 3Verify communication signal at outlet sensor where applicable
- 4Check harness continuity from ECM/module to outlet sensor
- 5Replace outlet sensor module only after circuit is proven
- 6Do not replace upstream sensor unless its circuit is independently proven faulty
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Compare outlet sensor data presence against upstream sensor data
- 2Verify power and ground at outlet sensor module connector
- 3Verify communication signal at outlet sensor where applicable
- 4Check harness continuity from ECM/module to outlet sensor
- 5Replace outlet sensor module only after circuit is proven
- 6Do not replace upstream sensor unless its circuit is independently proven faulty
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
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Summary
SPN 3226 FMI 3 is an outlet sensor supply and communication diagnosis. Upstream versus outlet data comparison is the first gate — paired NOx replacement is not indicated for one open outlet circuit.