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SPN 3226 FMI 2 — Downstream NOx sensor — erratic
SeriousAftertreatmentPower derate likely — plan to stop soon
Applies to: Detroit · Navistar · Paccar · Sae · Volvo
Downstream NOx sensor — erratic.
Don't risk it — get a diesel tech on the truck.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- NOx sensor aging
- Heater circuit
- Connector
- Heater
- Wiring
- NOx aging
First moves (roadside)
- Warmup test
- Connector check
- Allow warmup
- Check connectors
- Warmup
- Connector
FAQ
What does SPN 3226 FMI 2 mean?
Downstream NOx sensor — erratic.
How serious is SPN 3226 FMI 2?
Serious — Power derate likely — plan to stop soon.
What's the most likely cause of SPN 3226 FMI 2?
The most common cause is: NOx sensor aging. Other possibilities include Heater circuit; Connector; Heater; Wiring; NOx aging.
Can I keep driving with SPN 3226 FMI 2?
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Throwing this code right now?
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.