Truck Fault Codes

Mack MP8 SPN 3031 FMI 1 — DEF Tank Temperature Below Normal

SPN 3031 FMI 1 on the Mack MP8 means the DEF tank temperature is below the normal range. In cold weather, DEF freezes at -11 degrees C (12 degrees F), preventing DEF dosing. Allow engine warmup time for the DEF tank heater to thaw the DEF and monitor the temperature using Mack PTT. If the temperature does not rise after extended warmup, investigate the DEF tank heater coolant supply circuit.

Code Details

Structured details for SPN 3031 FMI 1
Display codeSPN 3031 FMI 1
SPN3031
FMI1
OEM codeNone listed
ManufacturerMack
SystemAftertreatment — SCR
ComponentDEF tank / DEF tank temperature sensor / DEF tank heater
Source addressUnknown or not applicable
Severitymedium
Review statussource backed
Source confidencemedium
Last reviewed2026-06-12

Plain-English Meaning

The Mack MP8 DEF system includes a tank heater that uses engine coolant to maintain the DEF above its freezing temperature during cold weather operation. When the DEF tank temperature sensor reads below the normal operational range, the ECM sets SPN 3031 FMI 1 to alert the driver. On MP8 trucks in Mack Anthem and Pinnacle linehaul applications operating in northern climates, overnight DEF freezing is a common cause of this fault during winter months. The DEF tank heater should thaw the frozen DEF during engine warmup if it is functioning properly. A failed heater or blocked coolant supply will prevent the DEF from thawing, keeping the DEF pump unable to deliver DEF to the dosing injector and causing SCR efficiency faults to develop as the NOx conversion is interrupted.

SPN 3031 is Aftertreatment 1 DEF Tank Temperature; FMI 1 indicates valid data below the normal operational range. On the Mack MP8 (GHG17 Volvo Group platform), the DEF tank heater uses engine coolant. Mack Premium Tech Tool (PTT) provides live DEF tank temperature data and DEF system status. Monitoring the temperature rise during warmup is the primary heater function check. If the temperature rises progressively during warmup, the heater is working and the DEF will thaw. If the temperature remains static near ambient despite extended engine operation, the heater coolant supply should be investigated.

Common Symptoms

  • DEF temperature warning on the instrument cluster in cold weather
  • SCR efficiency or DEF dosing faults alongside SPN 3031 FMI 1
  • Engine derate if DEF dosing is not restored and inducement advances

Possible Causes

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

  • DEF frozen from cold ambient overnight parking
  • DEF tank heater coolant supply blocked or disconnected
  • DEF tank heater element failure
  • DEF tank temperature sensor failure

First Checks

  • Assess overnight ambient temperatures and parking conditions
  • Allow extended warmup and monitor DEF temperature in Mack PTT
  • If temperature does not rise, check coolant supply lines to the DEF tank heater
  • Confirm coolant line temperature at the DEF tank heater connection is warm during engine operation
  • Test sensor reading plausibility against current ambient temperature

Can I Keep Driving?

The MP8 is driveable with SPN 3031 FMI 1 active. DEF dosing resumes after the DEF thaws. Repair the heater system if it is confirmed to be malfunctioning to prevent repeated cold-weather DEF system disruption.

SPN 4364 FMI 18Aftertreatment — SCR · SCR catalyst / DEF dosing system / NOx sensors / DEF quality

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

FAQ

Is SPN 3031 FMI 1 on the Mack MP8 a sign of DEF quality problems?

SPN 3031 FMI 1 indicates a temperature condition, not a DEF quality condition. DEF quality issues are indicated by separate fault codes such as the DEF quality fault or SCR efficiency codes. SPN 3031 FMI 1 is specifically about the DEF temperature being below the normal range. If the fault appears in cold weather and the DEF heater is confirmed to be functioning, the fault will clear as the temperature rises above the threshold after warmup, regardless of DEF quality.

Can the Mack MP8 run without DEF if SPN 3031 FMI 1 prevents dosing?

The MP8 cannot bypass the SCR inducement system simply because the DEF is frozen. If the DEF remains frozen and dosing continues to be unavailable, the ECM will initiate and advance the inducement sequence, eventually applying derates that limit speed and power. The inducement framework is mandated by EPA regulations and cannot be disabled in the field. Ensuring the DEF tank heater functions correctly in cold weather operations is essential to prevent inducement-related service interruptions.

Does the Mack MP8 share the DEF heating system design with the Volvo D13?

Yes. The Mack MP8 and Volvo D13 are closely related engines under the Volvo Group GHG17 platform and share a common DEF system architecture including the tank heater design. Diagnostic approaches for SPN 3031 FMI 1 are similar for both engines. The primary difference in diagnosis is the service tool — Mack MP8 uses Mack Premium Tech Tool (PTT) while the Volvo D13 uses Volvo Tech Tool or VCADS Pro for accessing DEF system live data and heater circuit diagnostics.