SPN 3226 FMI 2 — Downstream NOx sensor — erratic
What it means: Downstream NOx sensor — erratic.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Warmup test
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Phoenix, AZ
Closest first. Tell them SPN 3226 FMI 2 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 Phoenix, AZ · (623) 304-4187
- 0.0 mi away — based in Phoenix, AZ · (480) 458-7715
- 0.0 mi away — based in Phoenix, AZ · (480) 530-7347
Applies to: Detroit · Navistar · Paccar · Sae · Volvo
What SPN 3226 FMI 2 means
Downstream NOx sensor — erratic.
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
Downstream NOx sensor — erratic.
Serious — Power derate likely — plan to stop soon.
The most common cause is: NOx sensor aging. Other possibilities include Heater circuit; Connector; Heater; Wiring; NOx aging.
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 2, or share the same subsystem.
- Downstream NOx sensor — abnormal update rate (SPN 3226 FMI 9)
- Downstream NOx sensor — internal failure (SPN 3226 FMI 12)
- 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.