Code Details
| Display code | SPN 5246 FMI 31 |
|---|---|
| SPN | 5246 |
| FMI | 31 |
| OEM code | None listed |
| Manufacturer | International |
| 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 International A26 SCR system measures catalyst efficiency by comparing upstream and downstream NOx sensor readings. When the catalyst fails to convert sufficient NOx, the ECM logs low efficiency and begins the inducement countdown that eventually derates the engine if unresolved. On International LT Series, RH Series, and HX Series trucks with the A26, SCR efficiency is measured by the upstream and downstream NOx sensors. ServiceMaxx Pro provides both sensor readings and the calculated efficiency value. A large difference between calculated efficiency and the threshold — with correct DEF quality confirmed — points to SCR catalyst deterioration or a dosing system problem rather than a fluid issue. International dealers use ServiceMaxx to reset the inducement counter after the root cause is resolved.
SPN 5246 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 31 indicates the condition exists. On the International A26, the ECM monitors the upstream and downstream NOx sensors and calculates a conversion ratio. Causes include catalyst degradation, poisoning from contamination, chronic DEF quality issues, dosing faults, and NOx sensor drift. International Diamond Logic or JPRO provides live sensor data and service routines for diagnosis.
Common Symptoms
- Amber check engine lamp active
- SCR system warning on the International instrument cluster
- Inducement counter running in diagnostic tools or fleet telematics
- No immediate driveability change at initial fault set
- SCR efficiency warning on the International instrument cluster
- Possible companion DEF quality or dosing fault codes alongside SPN 5246
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 poisoning from oil or coolant contamination
- Chronic DEF quality issues reducing conversion performance
- DEF dosing system fault
- NOx sensor drift on either sensor
- SCR catalyst poisoned by engine oil or coolant contamination
- DEF dosing injector partially clogged reducing effective dose
First Checks
- Connect International Diamond Logic and review all active codes, NOx sensor live data, and DEF quality history
- Compare upstream and downstream NOx readings at operating temperature
- Test DEF dosing module function via diagnostic service routines
- Inspect exhaust system for contamination sources
- Connect ServiceMaxx and compare upstream and downstream NOx sensor readings to calculate actual SCR efficiency
- Verify DEF quality with a refractometer and confirm the dosing injector is operating at the correct dose rate
Can I Keep Driving?
The A26 can typically continue operating while this fault is active, but the inducement timer is running. Resolve the root cause before the derate threshold is reached to avoid operational disruption.
Related Codes
Related Lookup Pages
Sources
- International Trucks Service Support Public Resources International Trucks / Navistar International · oem · accessed 2026-06-11 · confidence medium
Source: International Trucks / Navistar International, International Trucks Service Support Public Resources. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - Navistar Engine Diagnostic Trouble Code Body Builder References Navistar / International Truck Body Builder · oem · accessed 2026-05-05 · confidence medium
Source: Navistar / International Truck Body Builder, Navistar Engine Diagnostic Trouble Code Body Builder References. 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
Is the A26 SCR system service procedure available to owner-operators, or only dealers?
Full SCR system diagnosis and inducement counter resets require International Diamond Logic or an equivalent authorized diagnostic tool. JPRO Fleet Diagnostics provides some level of access for owner-operators, but complete aftertreatment service functions may require a dealer-level connection to perform.
How does the International A26 SCR system compare to the MaxxForce 13 it replaced?
The A26 represents a significant redesign from the MaxxForce 13. The A26 uses a more conventional SCR-based aftertreatment system rather than the EGR-heavy approach of the MaxxForce 13, and it has a substantially better reliability record in industry surveys. The A26 aftertreatment is more typical of current industry practice.
Can a NOx sensor replacement fix SPN 5246 FMI 31 on the International A26 without catalyst replacement?
If the root cause is sensor drift rather than actual catalyst degradation, replacing the drifted sensor should resolve the efficiency fault. Use International Diamond Logic to verify sensor output against expected values before committing to catalyst replacement.