Truck Fault Codes

PACCAR MX-11 SPN 5246 FMI 31 �?SCR System Not Ready / Low Efficiency

SPN 5246 FMI 31 on a PACCAR MX-11 means the SCR catalyst conversion efficiency is below the regulatory threshold or the system cannot verify compliance. Common causes include DEF quality issues, a failed or plugged DEF dosing injector, a degraded SCR catalyst, or a faulty NOx sensor.

Code Details

Structured details for SPN 5246 FMI 31
Display codeSPN 5246 FMI 31
SPN5246
FMI31
OEM codeNone listed
ManufacturerPACCAR
SystemAftertreatment �?SCR system
ComponentSCR catalyst / DEF dosing system
Source addressUnknown or not applicable
Severitymedium
Review statussource backed
Source confidencemedium
Last reviewed2026-06-11

Plain-English Meaning

The SCR system is not converting enough NOx into nitrogen and water compared to what is required for emissions compliance. This is measured by comparing the upstream (before catalyst) and downstream (after catalyst) NOx sensor readings. When the gap is too small �?meaning not enough NOx is being converted �?this fault sets.

SPN 5246 is the Aftertreatment SCR Operator Inducement Severity parameter, and FMI 31 indicates a condition has been detected that requires operator awareness and may initiate inducement. On the PACCAR MX-11, the SCR efficiency is calculated from upstream and downstream NOx sensors. A low ratio triggers this fault. PACCAR DAVIE provides DEF dosing quantity, NOx sensor values, and SCR catalyst temperature data for a complete efficiency diagnosis.

Common Symptoms

  • SCR or DEF warning lamp active
  • Inducement counter visible in PACCAR DAVIE
  • Potential progressive derate if inducement condition persists

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

  • Plugged DEF dosing injector preventing urea from reaching the SCR catalyst
  • DEF quality below acceptable concentration for proper NOx reduction
  • SCR catalyst thermally damaged or chemically poisoned by contamination
  • Faulty upstream or downstream NOx sensor giving inaccurate efficiency calculation
  • DEF supply pump fault preventing adequate DEF delivery

First Checks

  • Connect PACCAR DAVIE and monitor DEF dosing quantity, SCR inlet temperature, and both NOx sensor readings simultaneously
  • Verify DEF quality using DAVIE sensor data and confirm the tank is filled with ISO 22241 certified DEF
  • Check DEF dosing injector operation �?DAVIE can command a dosing injector purge cycle
  • Inspect the DEF supply lines and pump module for blockages or leaks
  • If the catalyst temperature is consistently below 200°C during operation, consider whether duty cycle allows adequate SCR system warm-up

Can I Keep Driving?

At FMI 31, inducement may be accumulating or starting. The vehicle can still operate, but the countdown timer in DAVIE indicates how much time remains before a derate begins. Address the root cause without delay.

SPN 3364 FMI 1Aftertreatment �?DEF / SCR system · DEF quality sensor / SCR systemSPN 5246 FMI 31Aftertreatment �?SCR system · SCR catalyst / NOx sensors

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

Is SPN 5246 FMI 31 diagnosis the same on the PACCAR MX-11 as on the MX-13?

The diagnostic approach is essentially the same. Both engines use the same PACCAR-developed aftertreatment architecture and DAVIE diagnostic tool. The primary variable is the physical DEF dosing injector and catalyst sizing, which differs between the two displacement ratings but the underlying fault logic and sensor types are consistent.

How cold does the SCR catalyst need to be for SPN 5246 FMI 31 to occur on the MX-11?

SCR catalysts on the PACCAR MX-11 require an inlet temperature of approximately 200 to 220°C for the urea-based NOx reduction reactions to occur efficiently. Below this temperature, DEF dosing is inhibited and NOx conversion does not take place. Extended low-temperature operation �?such as short urban delivery routes in cold weather �?can lead to catalyst and injector deposits and potentially contribute to efficiency codes.

Can the DEF dosing injector on the PACCAR MX-11 be cleaned rather than replaced?

Some technicians successfully clean crystallized urea deposits from dosing injectors using approved cleaning procedures. The injector tip is particularly prone to urea buildup when the catalyst is frequently cooled and reheated. PACCAR DAVIE can command a purge cycle to help clear minor deposits. If DAVIE shows the injector is not dosing correctly despite a purge, replacement is the recommended solution.