Code Details
| Display code | SPN 3226 FMI 16 |
|---|---|
| SPN | 3226 |
| FMI | 16 |
| OEM code | None listed |
| Manufacturer | Detroit Diesel |
| System | Aftertreatment �?SCR system |
| Component | Upstream NOx sensor |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The upstream NOx sensor on the DD15 measures how much NOx leaves the engine before the SCR catalyst. When this reading is too high, it either means the engine is producing more NOx than normal (often from an EGR fault) or the sensor has drifted and is reading incorrectly. Either way, the reading affects the SCR efficiency calculation and can trigger additional aftertreatment codes.
SPN 3226 is Aftertreatment 1 Intake NOx. FMI 16 indicates a valid value above the normal operating range. On Detroit DD15 MCM2.0, the EGR system is critical for controlling combustion temperatures and NOx production. A partially stuck or failed EGR valve that reduces EGR flow will cause engine-out NOx to rise, directly increasing the upstream NOx sensor reading. DiagnosticLink can display both NOx sensors simultaneously and the EGR status for direct comparison.
Common Symptoms
- Check engine lamp active
- Possible SCR efficiency warning if the elevated upstream reading affects the efficiency calculation
- May accompany EGR valve fault codes
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- EGR valve fault reducing EGR flow and raising engine-out NOx
- EGR cooler bypass fault allowing high combustion temperatures
- Upstream NOx sensor drift reading high without actual elevated NOx
- Wiring or connector fault at the sensor causing electrical offset
First Checks
- Connect DiagnosticLink and check all active codes alongside SPN 3226 FMI 16
- Compare upstream and downstream NOx sensor readings live �?both high indicates actual high NOx; upstream high with downstream normal suggests sensor drift or EGR bypass
- Check EGR valve operation using DiagnosticLink bidirectional control to confirm it responds correctly
- Inspect the upstream NOx sensor harness and connector for damage or corrosion
Can I Keep Driving?
The vehicle is typically driveable. If EGR faults are contributing to real elevated NOx, DPF loading may accelerate. Resolve companion codes to restore normal EGR operation and NOx levels.
Related Codes
Related Lookup Pages
Sources
- Detroit Diesel Service and Diagnostic Resources — Public Reference Detroit Diesel (Daimler Truck North America) · oem · accessed 2026-06-11 · confidence medium
Source: Detroit Diesel (Daimler Truck North America), Detroit Diesel Service and Diagnostic Resources — Public Reference. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - DiagnosticLink Detroit Diesel Corporation · oem · accessed 2026-05-05 · confidence medium
Source: Detroit Diesel Corporation, DiagnosticLink. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
How do I distinguish an EGR fault from a sensor fault when diagnosing SPN 3226 FMI 16 on the DD15?
The most reliable indicator is companion EGR fault codes. If EGR valve position, flow, or cooler faults are also active, the elevated upstream NOx reading is likely real �?the EGR system is not reducing combustion temperatures adequately. If no EGR faults are present and the upstream NOx reading is consistently above range under all operating conditions including light load, sensor drift is more probable.
Can the EGR valve on the Detroit DD15 be cleaned to restore EGR flow and reduce NOx?
Yes, if the valve is sticking due to carbon buildup. DiagnosticLink can command the EGR valve through its full range to check for sticking, and cleaning the valve bore can restore responsiveness. However, if the valve actuator has failed electrically or the valve body is worn, replacement will be necessary.
Is the upstream NOx sensor location on the DD15 the same as on the DD13?
The DD13 and DD15 share a common aftertreatment architecture, but specific component locations may differ due to the different engine dimensions and cab/chassis fitments. In both cases, the upstream NOx sensor is typically located in the exhaust stream downstream of the turbocharger but before the DPF and SCR catalyst in the aftertreatment assembly.