Code Details
| Display code | SPN 5246 FMI 31 |
|---|---|
| SPN | 5246 |
| FMI | 31 |
| OEM code | None listed |
| Manufacturer | GM |
| 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 Duramax L5P SCR system measures catalyst efficiency by comparing upstream and downstream NOx levels. When conversion falls below the required threshold, the ECM logs this code and starts the inducement countdown. This code can appear at high mileage or after DEF quality issues have affected the catalyst. On Chevrolet Silverado 2500/3500 HD and GMC Sierra 2500/3500 HD trucks with the Duramax L5P, SCR efficiency is tracked by the ECM using upstream and downstream NOx sensor data. ACDelco TDS and GM TIS2Web provide access to the SCR efficiency metrics and the inducement counter status. The L5P uses a highly capable SCR system, so poor DEF quality is the most common cause of SPN 5246 FMI 31 in fleet service — catalyst failure is less frequent but possible after high mileage or improper DEF contamination.
SPN 5246 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 31 indicates the condition exists. On the Duramax L5P, the ECM monitors the NOx conversion ratio over defined drive cycles. Causes include catalyst degradation from age, heat, or contamination; DEF quality issues; dosing faults; and NOx sensor drift. GM GDS2 provides live NOx sensor data, dosing history, and service functions for diagnosis.
Common Symptoms
- Check engine lamp active
- SCR system warning in the Driver Information Center
- Inducement counter running in GDS2 or telematics
- No immediate driveability change at initial fault set
- SCR efficiency warning on the Silverado or Sierra HD gauge cluster
- Inducement derate will follow if the root cause is not resolved
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- SCR catalyst 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
- SCR catalyst degraded from high mileage or heat cycling
- Dosing injector partially clogged reducing effective DEF dose to the catalyst
First Checks
- Connect GM GDS2 and review 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 GDS2 service routines
- Inspect exhaust system for contamination sources
- Verify DEF quality with a refractometer — rule out DEF as the root cause first
- Connect ACDelco TDS and review NOx sensor upstream/downstream data and SCR efficiency calculation
Can I Keep Driving?
The Duramax L5P can typically continue operating with this fault active, but the inducement timer is running. For heavy-duty towing use, address this promptly to avoid a derate during a towing trip.
Related Codes
Related Lookup Pages
Sources
- GM Trucks Service Support — Duramax Diesel Resources General Motors LLC · oem · accessed 2026-06-11 · confidence medium
Source: General Motors LLC, GM Trucks Service Support — Duramax Diesel Resources. 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 Duramax L5P always require SCR catalyst replacement?
Not necessarily. A systematic diagnosis using GDS2 to check NOx sensor accuracy, DEF dosing output, and DEF quality history often identifies a less expensive root cause. Sensor drift or dosing system faults are relatively common causes that are less expensive to fix than catalyst replacement.
Is the Duramax L5P SCR catalyst the same unit as used in the LML or LGH?
The L5P (2017+) uses a new SCR-first aftertreatment architecture that differs from earlier Duramax platforms. The SCR catalyst and DPF are different components from the LML and LGH. Service procedures for the L5P are specific to its aftertreatment layout.
Can low-speed towing in city traffic cause recurring SPN 5246 FMI 31 on the L5P?
Repeated low-temperature SCR operation from city towing cycles can affect catalyst efficiency readings during those cold periods. However, if the fault persists consistently across multiple drive cycles at normal operating temperature, a hardware issue is the more likely cause.