SPN 3226 FMI 12 — Downstream NOx sensor — internal failure
What it means: Downstream NOx sensor — internal failure.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Replace
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Indianapolis, IN
Closest first. Tell them SPN 3226 FMI 12 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Indianapolis, IN · (317) 789-5002
- 0.0 mi away — based in Indianapolis, IN · (317) 300-7000
- 0.0 mi away — based in Indianapolis, IN · (317) 646-8208
Applies to: Sae
What SPN 3226 FMI 12 means
Downstream NOx sensor — internal failure.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Sensor self-reported fail
First moves (roadside)
- Replace
FAQ
Downstream NOx sensor — internal failure.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Sensor self-reported fail.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Related codes
Codes that commonly appear alongside SPN 3226 FMI 12, or share the same subsystem.
- Downstream NOx sensor — erratic (SPN 3226 FMI 2)
- Downstream NOx sensor — abnormal update rate (SPN 3226 FMI 9)
- SCR Operator Inducement — Severe Derate Active (SPN 5246 FMI 0)
- Aftertreatment 1 Diesel Particulate Filter Differential Pressure - Data Valid But Above Normal Operating Range, Most Severe Level (DPF severely restricted / plugged) (SPN 3251 FMI 0)
- Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect) (SPN 4364 FMI 1)
- DPF Differential Pressure — Data Erratic (SPN 3251 FMI 2)
- DEF Dosing Unit — Not Responding Properly (SPN 3361 FMI 7)
- SCR NOx Catalyst Efficiency Below Threshold — Bank 1 (P20EE)
Same symptom: DEF / SCR fault
Other codes in the DEF and SCR chain — the ones that end in an inducement countdown.
- Reductant System Performance Bank 1 (P204F) · Aftertreatment & Emissions
- Reductant Quality Deviation (P20EA) · Aftertreatment & Emissions
- DEF tank level low (SPN 1761 FMI 18) · Aftertreatment & Emissions
- DEF tank temp sensor — voltage high (SPN 3031 FMI 3) · Aftertreatment & Emissions
- Upstream NOx sensor — internal failure reported (SPN 3216 FMI 12) · Aftertreatment & Emissions
- SCR inlet NOx sensor — erratic (SPN 3216 FMI 2) · Aftertreatment & Emissions
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.