Detailed guide for SPN 3226 FMI 20
Back to top ↑Outlet NOx drift causes the system to believe SCR conversion is failing even when dosing is fine.\n\nRule out exhaust leaks before replacing sensors.
Jump to
Share ↓SPN 3226 FMI 20Severity: Warning
SPN 3226 FMI 20: Aftertreatment outlet NOx sensor drift – SCR efficiency calculations unreliable
Quick Answer
Aftertreatment outlet NOx sensor drift – SCR efficiency calculations unreliable
Code
SPN 3226 FMI 20
Severity
Warning
Applies to
Check OEM application
Can I drive?
Driving suitability is not established by this code alone.
Common Symptoms
- SCR efficiency faults later
- Intermittent aftertreatment light
- Derate risk if it escalates
Likely Causes
- Sensor drift with age
- Exhaust leak near sensor
- Contamination from soot
Common Fixes
- Check for exhaust leaks at clamps/gaskets
- Compare inlet vs outlet NOx behavior
- Replace sensor if drift persists
Full technical sections↓
What does SPN 3226 FMI 20 mean?
Back to top ↑Aftertreatment outlet NOx sensor drift – SCR efficiency calculations unreliable
BigRigFaults Community
Community discussions related to SPN 3226 FMI 20
Compare symptoms, repairs and real-world experience from drivers, technicians and owner-operators.
Fault Codes / SPN / FMI
SPN 1590 FMI 19 - Meaning, Causes & Diagnostic Discussion
0 replies7 views
Fault Codes / SPN / FMI
SPN 4077 FMI 14 - Meaning, Causes & Diagnostic Discussion
0 replies11 views
Fault Codes / SPN / FMI
SPN 84 FMI 21 - Meaning, Causes & Diagnostic Discussion
0 replies8 views
Fault Codes / SPN / FMI
SPN 1071 FMI 5 - Meaning, Causes & Diagnostic Discussion
0 replies7 views
Still stuck with SPN 3226 FMI 20?
Ask the community and include your truck year, make, engine, active codes, symptoms and recent repairs so people can give useful answers instead of performing mechanical astrology.
Ask the Community →Related fault codes
No related faults found yet.
Mechanic community notes for SPN 3226 FMI 20
Back to top ↑Real-world tips from technicians. Submissions are moderated to keep spam and “my cousin fixed it with duct tape” content out.
Share your fix / advice
Keep it useful: symptoms, what you checked, what fixed it, and whether the code stayed inactive after a drive cycle.