Truck Fault Codes

Mack MP8 SPN 5246 FMI 31 �?SCR Conversion Efficiency Low

SPN 5246 FMI 31 on a Mack MP8 indicates the SCR catalyst is not converting NOx at the required efficiency. Common causes include catalyst degradation, DEF quality issues, dosing system faults, or NOx sensor drift.

Code Details

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

Plain-English Meaning

The Mack MP8 SCR system, shared with the Volvo D13, measures efficiency by comparing NOx at the catalyst inlet and outlet. When conversion falls below the required threshold, the ECM flags low efficiency and starts the inducement countdown. On Mack Anthem, Pinnacle, and Granite trucks, the SCR inducement system tracks two things independently: DEF quality and SCR efficiency. SPN 5246 FMI 31 means the efficiency calculation is below threshold — which can be caused by poor DEF quality, a failing catalyst, or a dosing injector that is not delivering the commanded dose. VCADS Pro displays the upstream and downstream NOx sensor readings simultaneously, which is the key diagnostic step to assess catalyst effectiveness versus dosing adequacy.

SPN 5246 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 31 indicates the condition exists on the Mack MP8 EMS platform. The ECM calculates efficiency from the upstream and downstream NOx sensor ratio. Diagnostic access via Mack service tools or JPRO provides live NOx sensor data, DEF dosing history, and service test functions identical to VCADS Pro used on the Volvo D13 platform.

Common Symptoms

  • Amber check engine lamp active
  • SCR system warning on the Mack instrument cluster
  • Inducement counter running in Mack service tools 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 degraded from high mileage or thermal damage
  • Catalyst poisoning from oil or coolant contamination in the exhaust
  • Chronic DEF quality issues reducing conversion performance
  • DEF dosing system fault causing insufficient reagent delivery
  • NOx sensor drift on either the upstream or downstream sensor

First Checks

  • Connect Mack diagnostic software 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 Mack service routines
  • Inspect exhaust system for contamination sources
  • Compare upstream and downstream NOx sensor readings in VCADS Pro to assess catalyst efficiency
  • Confirm the dosing injector is delivering commanded DEF dose in VCADS Pro live data

Can I Keep Driving?

The Mack MP8 can typically continue operating with this fault active, but the inducement timer is running. Diagnose and resolve before the derate threshold is reached.

SPN 5246 FMI 31Aftertreatment �?SCR system · SCR catalyst / NOx sensorsSPN 3364 FMI 1Aftertreatment �?DEF system · DEF quality / concentration sensorSPN 1569 FMI 31Engine protection / aftertreatment · Engine control module �?protection strategy

Related Lookup Pages

Sources

  • Mack Trucks Service Support Public Resources Mack Trucks Inc. · oem · accessed 2026-06-11 · confidence medium

    Source: Mack Trucks Inc., Mack Trucks Service Support Public Resources. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • Volvo Trucks / Mack Trucks Technical Support Volvo Trucks North America / Mack Trucks Inc. · oem · accessed 2026-06-10 · confidence medium

    Source: Volvo Trucks North America / Mack Trucks Inc., Volvo Trucks / Mack Trucks Technical Support. 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 Mack MP8 SCR efficiency diagnosis the same process as the Volvo D13?

Yes �?the MP8 and D13 share the same platform, so the diagnostic approach is identical: check NOx sensor accuracy, DEF dosing performance, and DEF quality history before considering catalyst replacement. The difference is in the diagnostic tool �?Mack-branded tools or JPRO vs. VCADS Pro.

Can a Mack dealer reset the SPN 5246 FMI 31 inducement counter on an MP8?

After identifying and correcting the root cause �?whether a sensor, dosing issue, or genuine catalyst problem �?a Mack dealer can reset the fault codes and inducement counter using Mack service tools. The ECM verifies system performance over a drive cycle before fully resetting the counter.

How does the MP8 SCR catalyst age compared to other heavy-duty engines?

SCR catalyst service life on the MP8 is comparable to other Class 8 heavy-duty engines �?typically several hundred thousand miles under normal operating conditions. Aggressive duty cycles, chronic DEF quality issues, or contamination events can accelerate degradation. Using quality DEF, maintaining proper oil change intervals, and addressing related faults promptly helps maximize catalyst life.