Code Details
| Display code | SPN 5246 FMI 31 |
|---|---|
| SPN | 5246 |
| FMI | 31 |
| OEM code | None listed |
| Manufacturer | Detroit Diesel |
| System | Aftertreatment �?SCR system |
| Component | SCR catalyst / NOx sensors |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The DD15 SCR system measures efficiency by comparing NOx levels entering and exiting the catalyst. When conversion falls below the required threshold, the MCM flags this code and starts the inducement countdown. Long-haul DD15 applications accumulate inducement distance quickly at highway speeds, making early diagnosis important. On Freightliner Cascadia and Western Star trucks with the DD15, the ACM manages SCR efficiency monitoring and the inducement counter. DiagnosticLink provides a side-by-side view of upstream and downstream NOx sensor readings plus the calculated efficiency percentage. An SCR catalyst that is sulfur-poisoned from low-quality diesel can be partially regenerated by a desulfation cycle, which DiagnosticLink can initiate. A physically damaged or aged catalyst that cannot meet the efficiency threshold requires replacement.
SPN 5246 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 31 indicates the condition exists on the Detroit MCM2.0 platform. The MCM calculates conversion efficiency from the upstream and downstream NOx sensor ratio. Degradation causes include high-mileage catalyst wear, sulfur poisoning, thermal damage, chronic DEF quality issues, and NOx sensor drift. DiagnosticLink provides access to live NOx sensor data, dosing history, and DEF quality records needed for a systematic diagnosis.
Common Symptoms
- Amber check engine lamp active
- SCR system performance warning on the dash display
- Inducement counter running in DiagnosticLink or telematics
- No immediate driveability change at fault set
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- SCR catalyst substrate degraded from high mileage or thermal damage
- Catalyst poisoned by oil or coolant contamination in the exhaust
- Chronic DEF quality issues reducing conversion performance over time
- DEF dosing system fault causing insufficient reagent delivery
- NOx sensor drift on the upstream or downstream sensor
First Checks
- Connect DiagnosticLink and review all active codes, NOx sensor live data, and DEF quality and dosing history
- Compare upstream and downstream NOx sensor readings at normal operating temperature
- Test DEF dosing module function using DiagnosticLink service routines
- Inspect exhaust system for contamination sources
- Connect DiagnosticLink and review the SCR efficiency percentage alongside both NOx sensor readings
- If catalyst is suspected of sulfur poisoning, check fuel sulfur content and initiate a DiagnosticLink desulfation cycle
Can I Keep Driving?
The DD15 can typically continue operating with this fault active, but at long-haul highway speeds the inducement distance accumulates rapidly. Diagnose and resolve before the derate threshold is reached to avoid disruption during a trip.
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 quickly does the DD15 accumulate inducement distance at highway speeds?
At typical long-haul speeds of 60�?0 mph, a DD15 can accumulate several hundred miles per day. If the inducement counter shows only a few hundred miles remaining before the derate threshold, a driver on a long run may not make it to a service facility before the derate activates. Check the counter early and plan accordingly.
Does SPN 5246 FMI 31 on the DD15 always require catalyst replacement?
Not always. A systematic diagnosis using DiagnosticLink to check NOx sensor accuracy, DEF dosing output, and DEF quality history often identifies a less expensive root cause before committing to a catalyst replacement. Sensor drift and dosing faults are relatively common and less expensive to fix than catalyst replacement.
What should I check first if both SPN 5246 FMI 31 and SPN 3364 FMI 1 are active together on the DD15?
Address the DEF quality issue first �?SPN 3364 FMI 1 (low DEF concentration) can directly cause SPN 5246 FMI 31 (low SCR efficiency) if the catalyst has been starved of adequate reagent. Correcting the DEF quality and allowing the system to operate normally may resolve the efficiency fault without further catalyst service.