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 DD13 SCR system injects DEF into hot exhaust to convert NOx to nitrogen and water. When the downstream NOx sensor shows too much NOx exiting the catalyst, the MCM flags low efficiency and begins the inducement countdown. Continued operation without resolving the issue leads to progressive derates. On Freightliner Cascadia trucks with the DD13, the NOx sensors upstream and downstream of the SCR catalyst measure SCR efficiency in real time. DiagnosticLink displays both NOx sensor readings simultaneously, and the efficiency calculation is derived from the ratio. If the upstream NOx sensor is reading lower than expected — from EGR dilution of exhaust NOx before the SCR — the calculated efficiency may appear artificially high. DiagnosticLink can force the EGR valve closed during a diagnostic test cycle to isolate the SCR efficiency reading from EGR dilution effects.
SPN 5246 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 31 indicates a condition exists �?on Detroit DDEC10/MCM2.0, this fault activates when the measured conversion ratio falls below the calibrated threshold over a defined operating window. The MCM monitors both the upstream and downstream NOx sensors and calculates an efficiency ratio. Causes include catalyst degradation from age, heat, or contamination; DEF quality issues; dosing system faults; and NOx sensor drift on either sensor.
Common Symptoms
- Amber check engine lamp active
- SCR system warning on the dash display
- Inducement distance 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 event
- Catalyst poisoning from oil or coolant contamination in the exhaust
- Chronic DEF quality issues reducing long-term conversion performance
- 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, DEF quality history, and dosing system status
- Compare upstream and downstream NOx readings at operating temperature
- Check DEF dosing module function using DiagnosticLink service routines
- Inspect exhaust system for contamination sources that could poison the catalyst
- Connect DiagnosticLink and compare upstream and downstream NOx sensor readings simultaneously
- Use DiagnosticLink to run the SCR efficiency diagnostic test with the EGR valve forced closed
Can I Keep Driving?
The DD13 can typically continue operating with this fault active, but the inducement timer is running. Diagnose before the derate threshold is reached to avoid operational disruption.
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
Does SPN 5246 FMI 31 on the DD13 mean the SCR catalyst needs to be replaced?
Not necessarily. The efficiency calculation depends on NOx sensor accuracy and DEF dosing performance as well as the actual catalyst condition. A systematic check of sensors and dosing system often identifies the root cause before catalyst replacement is needed.
Can the DD13 SCR catalyst be cleaned, or does it need to be replaced if degraded?
SCR catalyst cleaning is not a standard field service procedure �?unlike the DPF, the SCR catalyst does not accumulate soot that can be burned off. If the SCR substrate is genuinely degraded or poisoned, replacement is typically required. However, contamination from a one-time event (such as an oil leak into the exhaust) may leave some catalyst function intact.
How does DiagnosticLink help diagnose SPN 5246 FMI 31 on the DD13?
DiagnosticLink provides live NOx sensor values, DEF dosing system status and fault history, catalyst inlet and outlet temperatures, and DEF quality history. Using these data points together allows a technician to distinguish between a measurement error (sensor drift), a delivery error (dosing fault), and a true catalyst efficiency problem.