Truck Fault Codes

PACCAR MX-13 SPN 3031 FMI 1 — DEF Tank Temperature Below Normal

SPN 3031 FMI 1 on the PACCAR MX-13 means the DEF tank temperature is below the normal range, most commonly from DEF freezing in cold weather. Allow engine warmup time for the tank heater to thaw frozen DEF and use PACCAR ESA to monitor the temperature rise. If the temperature does not rise during extended operation, investigate the DEF tank heater circuit.

Code Details

Structured details for SPN 3031 FMI 1
Display codeSPN 3031 FMI 1
SPN3031
FMI1
OEM codeNone listed
ManufacturerPACCAR
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 PACCAR MX-13 aftertreatment system includes a DEF tank heater that prevents the DEF from freezing during cold weather operation. DEF freezes at approximately -11 degrees Celsius (12 degrees Fahrenheit), and frozen DEF cannot be pumped to the dosing injector, interrupting SCR function. When the DEF tank temperature sensor reads below the normal operational range, the ACM sets SPN 3031 FMI 1. In cold weather operations with the MX-13 in Kenworth T680 and Peterbilt 579 trucks, overnight freezing of the DEF is a common cause of this fault. The tank heater system is designed to thaw the DEF during engine warmup after a cold start. If the heater has failed, the DEF temperature will not rise during warmup, DEF dosing will remain interrupted, and SCR efficiency faults will develop as the aftertreatment system detects the absence of NOx conversion.

SPN 3031 is Aftertreatment 1 DEF Tank Temperature; FMI 1 indicates valid data below the normal operational range. On the PACCAR MX-13, the DEF tank heater uses engine coolant to warm the DEF. PACCAR ESA provides live DEF tank temperature data and DEF system status. The temperature trend during warmup is the primary diagnostic indicator — temperature rising during warmup indicates a functioning heater; temperature remaining near ambient indicates a heater system fault. ESA can display the heater circuit status and related DEF system parameters.

Common Symptoms

  • DEF temperature warning in cold weather
  • SCR efficiency or DEF system faults developing alongside SPN 3031 FMI 1
  • Engine derate if DEF dosing is interrupted and the inducement timer advances

Possible Causes

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

  • DEF frozen from cold ambient temperatures
  • DEF tank heater coolant supply circuit failure
  • DEF tank temperature sensor failure

First Checks

  • Confirm overnight ambient temperature and assess likelihood of DEF freezing
  • Allow extended warmup and monitor DEF tank temperature in PACCAR ESA
  • If temperature does not rise, inspect coolant supply to the DEF tank heater
  • Test sensor plausibility against ambient and coolant temperatures
  • Check for related SCR efficiency fault codes confirming DEF dosing interruption

Can I Keep Driving?

The MX-13 is driveable with SPN 3031 FMI 1. DEF dosing will resume after the DEF thaws. If the heater has failed, repair promptly to prevent recurring SCR disruption in cold weather.

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

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

FAQ

How long should I idle the MX-13 after a cold start before expecting frozen DEF to thaw?

With a functioning DEF tank heater, the DEF on a MX-13 cold-soaked to temperatures near the freezing point will typically thaw within 30 to 60 minutes of engine operation. In more extreme cold conditions (-20 degrees C or below) or with a DEF tank that has been completely frozen solid, thaw time may be longer. PACCAR ESA live data can show the DEF temperature rising toward normal, which confirms the thaw is progressing and dosing will resume.

Can wrong DEF level cause SPN 3031 FMI 1 on the MX-13?

A very low DEF level can affect the temperature sensor reading if the sensor is positioned in the tank at a level that is no longer submerged in DEF — the sensor may read the air temperature in the empty space above the DEF rather than the DEF temperature itself. This can produce a below-normal reading even when the actual DEF in the tank is not frozen. Checking the DEF level and ensuring the tank has adequate fluid is a useful early step when SPN 3031 FMI 1 appears.

Do PACCAR MX-13 trucks in warm climates ever get SPN 3031 FMI 1?

SPN 3031 FMI 1 in warm climates where DEF freezing is not plausible is most likely caused by a DEF tank temperature sensor failure producing a false below-normal reading. Comparing the sensor reading against the ambient temperature and the engine operating temperature in PACCAR ESA can identify a sensor that is reading significantly colder than ambient when no actual cold condition exists. Sensor replacement is the appropriate repair when a false reading is confirmed.