Code Details
| Display code | SPN 5246 FMI 31 |
|---|---|
| SPN | 5246 |
| FMI | 31 |
| OEM code | None listed |
| Manufacturer | PACCAR |
| 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 PACCAR MX-13 SCR system monitors conversion efficiency using upstream and downstream NOx sensors. When the catalyst fails to convert enough NOx, the ECU flags low efficiency and begins the inducement countdown. Long-haul applications accumulate inducement distance quickly at highway speeds. On Kenworth and Peterbilt trucks with the MX-13, SCR efficiency is calculated from the upstream and downstream NOx sensor ratio. DAVIE4 and PACCAR ESA both display this ratio and the individual sensor readings. A downstream NOx sensor that is contaminated with DEF crystalline residue may read artificially low, making SCR efficiency appear artificially high and masking a genuine dosing problem. Alternatively, a failed downstream sensor can read stuck-high, triggering SPN 5246 FMI 31 with a catalyst that is actually functioning normally.
SPN 5246 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 31 indicates the condition exists. On the PACCAR MX-13, the ECU calculates a conversion ratio from upstream and downstream NOx sensor readings over a defined operating window. When the ratio falls below the calibrated threshold, this fault is logged. The diagnostic approach mirrors other heavy-duty platforms: check NOx sensor accuracy, DEF dosing system function, and DEF quality history before committing to catalyst replacement.
Common Symptoms
- Amber check engine lamp active
- SCR system warning on the Kenworth or Peterbilt dash display
- Inducement distance counter in telematics or instrument cluster
- No immediate driveability change at initial 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
- Chronic DEF quality issues reducing long-term efficiency
- DEF dosing system fault causing insufficient reagent delivery
- NOx sensor drift on either the upstream or downstream sensor
First Checks
- Connect PACCAR DAVIE and review all active codes, NOx sensor live data, and DEF quality and dosing history
- Compare upstream and downstream NOx readings at normal operating temperature
- Test DEF dosing module function via PACCAR diagnostic service routines
- Inspect exhaust system for contamination sources that could affect catalyst performance
- Inspect the downstream NOx sensor bung for DEF crystalline residue that could affect sensor accuracy
- Compare NOx sensor readings in DAVIE4 to confirm both sensors are responding to exhaust gas changes
Can I Keep Driving?
The MX-13 can typically continue operating with this fault active, but the inducement timer is running and distance accumulates quickly on highway duty cycles. Diagnose and resolve the root cause before the derate threshold is reached.
Related Codes
Related Lookup Pages
Sources
- PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources PACCAR Inc. · oem · accessed 2026-06-11 · confidence medium
Source: PACCAR Inc., PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - PACCAR Engine Technical Services PACCAR Inc. · oem · accessed 2026-06-10 · confidence medium
Source: PACCAR Inc., PACCAR Engine Technical Services. 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 PACCAR MX-13 require dealer-level tools to diagnose?
Full diagnosis requires PACCAR DAVIE software, which provides NOx sensor live data, dosing history, and service test functions. Some data may be accessible via generic J1939 tools, but DAVIE provides the full set of parameters needed for an accurate diagnosis without unnecessary parts replacement.
Is the PACCAR MX-13 SCR system the same as used on DAF trucks?
The PACCAR MX-13 is derived from the DAF MX-13 platform and shares significant design elements. However, North American MX-13 applications have calibration differences for U.S. and Canadian emissions regulations. Service procedures and component part numbers may differ; always reference PACCAR-specific documentation for North American trucks.
Can a new downstream NOx sensor resolve SPN 5246 FMI 31 on the MX-13?
If sensor drift is the root cause, replacing the downstream NOx sensor can resolve the fault. Use DAVIE to monitor the new sensor output across a drive cycle to confirm the efficiency ratio returns to within the acceptable range. If the fault persists after sensor replacement, deeper catalyst or dosing system investigation is warranted.