Code Details
| Display code | SPN 5246 FMI 31 |
|---|---|
| SPN | 5246 |
| FMI | 31 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Aftertreatment �?SCR |
| Component | SCR catalyst / NOx sensors / DEF system |
| Source address | Unknown or not applicable |
| Severity | high |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-12 |
Plain-English Meaning
The ISB 6.7 SCR system converts NOx (nitrogen oxides) from the exhaust using DEF and a catalyst. SPN 5246 FMI 31 means the ECM's NOx sensors detected that the catalyst is not converting enough NOx �?the outlet NOx level is too high relative to the inlet. In medium-duty Freightliner M2, Kenworth T270/T370, and Peterbilt 325 trucks, this code is commonly triggered by DEF quality issues, a contaminated or aged SCR catalyst, or a DEF dosing system fault.
SPN 5246 FMI 31 is SCR Conversion Efficiency below the compliance threshold. On Cummins ISB 6.7 CM2350, the ECM compares NOx sensor readings at the SCR inlet and outlet to calculate conversion efficiency. If the ratio falls below the calibration threshold, SPN 5246 is set and the inducement sequence begins. Insite displays NOx inlet and outlet readings, DEF consumption rate, and dosing system parameters.
Common Symptoms
- Check engine lamp and possibly a STOP ENGINE or emissions warning on the dash
- Engine derate if the efficiency fault is not resolved �?distance-based escalation
- Possibly associated DEF quality or dosing fault codes
- Vehicles operating in cold weather may show this if DEF dosing was compromised
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF quality below specification �?diluted, contaminated, or incorrectly blended fluid
- SCR catalyst aged beyond service life or contaminated with coolant or oil
- DEF dosing injector fault causing reduced or erratic DEF delivery
- NOx sensor (inlet or outlet) fault giving incorrect conversion calculation
- DEF supply system fault (pump, filter, lines) reducing actual DEF delivery
First Checks
- Connect Cummins Insite and check the NOx inlet and outlet live readings �?a large ratio difference (high outlet NOx) confirms low conversion
- Test DEF quality with a refractometer to confirm urea concentration is within the 31.8�?3.2% specification
- Check for companion DEF system fault codes (SPN 3364, SPN 3361, SPN 3480)
- Review DEF consumption rate in Insite �?abnormally low consumption may indicate a dosing fault
- If DEF quality and dosing are confirmed correct, evaluate SCR catalyst condition or age
Can I Keep Driving?
The ISB 6.7 inducement sequence will reduce engine power if this fault is not resolved. Address before escalation reaches severe derate.
Related Codes
Related Lookup Pages
Sources
- Cummins INSITE Service Tool — Public Reference Documentation Cummins Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Cummins Inc., Cummins INSITE Service Tool — Public Reference Documentation. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - QuickServe Online Cummins Inc. · oem · accessed 2026-05-05 · confidence medium
Source: Cummins Inc., QuickServe Online. 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
Does the Cummins ISB 6.7 in a medium-duty truck have the same DEF and SCR system as the ISX15?
The ISB 6.7 and ISX15/X15 both use Cummins SCR aftertreatment with DEF, but the ISB 6.7 is a lighter-duty engine with a smaller, simpler aftertreatment package sized for medium-duty applications. The diagnostic approach is similar �?Insite, DEF quality testing, NOx sensor comparison �?but component specifications and service procedures are specific to the ISB 6.7 platform.
How often does the ISB 6.7 SCR catalyst need replacement in a medium-duty truck?
SCR catalyst service life depends heavily on DEF quality, oil consumption, and operating conditions. A well-maintained ISB 6.7 with proper DEF can see SCR catalyst life well beyond 200,000 miles. Poor DEF quality, oil burning, and coolant leaks that contaminate the aftertreatment accelerate catalyst degradation significantly.
Can SPN 5246 FMI 31 on the ISB 6.7 be cleared without a repair?
The code can be cleared using Insite, but if the underlying cause has not been corrected, the fault will return within a short drive cycle. The inducement sequence on the ISB 6.7 tracks cumulative fault hours �?simply clearing the code resets the active fault but does not reset the inducement counter. A proper repair and Insite reset is required to fully restore normal operation.