Code Details
| Display code | SPN 3031 FMI 1 |
|---|---|
| SPN | 3031 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Aftertreatment — SCR |
| Component | DEF tank / DEF tank temperature sensor / DEF tank heater |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-12 |
Plain-English Meaning
The Cummins X12 is a 12-liter engine used in vocational and linehaul trucks that relies on a properly functioning DEF system for SCR emissions compliance. The X12 DEF tank includes a heating element that uses engine coolant to maintain the DEF above its freezing point (-11 degrees C / 12 degrees F) during cold weather operation. When the DEF tank temperature sensor reads below the normal operational range, the ECM sets SPN 3031 FMI 1. During cold winter starts, it is common for the DEF to be partially or fully frozen if the truck was parked outdoors in freezing temperatures. The tank heater is designed to thaw the DEF during the engine warmup period following a cold start. If the heater is not functioning — from a blocked coolant supply line, a failed heater element, or a disconnected hose — the DEF will remain frozen, preventing DEF dosing and causing SCR efficiency faults to develop over time.
SPN 3031 is Aftertreatment 1 DEF Tank Temperature; FMI 1 indicates valid data below the normal operational range. On the Cummins X12 (CM2350 X105), the DEF tank heater uses engine coolant. Cummins INSITE provides live DEF tank temperature data and heater system status. The key diagnostic indicator is whether the DEF temperature rises progressively during engine warmup — a rising temperature confirms the heater is working, while a static temperature near ambient after extended warmup indicates a heater circuit fault. INSITE can also display any SCR efficiency or DEF dosing fault codes that confirm DEF delivery was interrupted while the DEF was frozen.
Common Symptoms
- DEF temperature or SCR system warning lamp during cold weather operation
- SCR efficiency or DEF dosing fault codes appearing alongside SPN 3031 FMI 1
- Engine derate if DEF remains unavailable and inducement timer advances
- DEF pump inactivity during cold start until DEF thaws
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF frozen from overnight cold soak at temperatures below -11 degrees C
- DEF tank heater coolant supply blocked, kinked, or disconnected
- DEF tank heater element failure
- DEF tank temperature sensor failure producing a false below-normal reading
First Checks
- Check whether overnight ambient temperatures were below the DEF freezing point
- Allow extended engine warmup and monitor DEF tank temperature in Cummins INSITE
- If the temperature is not rising during warmup, inspect coolant supply lines to the DEF tank heater for blockage or disconnection
- Verify the coolant supply lines at the DEF tank heater connection are warm during engine operation
- Compare the sensor reading in INSITE against the ambient temperature for plausibility in warm-weather cases
Can I Keep Driving?
The X12 is driveable with SPN 3031 FMI 1 active. DEF dosing resumes once the DEF thaws and the temperature returns to the normal range. If the heater has failed, repair is required to prevent recurring SCR disruption in cold weather.
Related Lookup Pages
Sources
- Cummins INSITE Service Tool — Public Reference Documentation Cummins Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Cummins Inc., Cummins INSITE Service Tool — Public Reference Documentation. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - QuickServe Online Cummins Inc. · oem · accessed 2026-05-05 · confidence medium
Source: Cummins Inc., QuickServe Online. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Is the Cummins X12 DEF tank heater the same design as the ISX15 heater?
The Cummins X12 and ISX15 share the Cummins CM2350 emissions platform and use similar aftertreatment system architectures, including coolant-heated DEF tank systems. The DEF tank assembly design may differ between engine displacements and vehicle integrations, but the fundamental heating mechanism — engine coolant circulating through a heat exchanger in the DEF tank — is common across the CM2350 family. INSITE DEF heater diagnostics and temperature monitoring work the same way for both engines.
Can SPN 3031 FMI 1 on the Cummins X12 be caused by a low DEF level?
Yes. If the DEF tank level is very low and the temperature sensor is mounted in the upper portion of the tank, the sensor tip may be exposed to air rather than submerged in DEF. Air temperature in the tank can be lower than the DEF temperature, causing the sensor to read a below-normal value that does not accurately reflect the actual DEF temperature at the pickup location. Filling the DEF tank to a proper level and monitoring whether SPN 3031 FMI 1 recurs is a useful diagnostic step when the fault appears in conditions where DEF freezing is not expected.
What coolant specification flows through the Cummins X12 DEF tank heater?
The X12 DEF tank heater uses the same engine coolant that circulates through the main cooling system — a Nitrite-Free Extended Life Coolant (ELC) as specified for the Cummins X12. The coolant flows through the DEF tank heater circuit when the DEF temperature is below the target level. The heater circuit does not require a separate coolant type; it is a branch of the main cooling circuit. Ensuring the main cooling system is filled with the correct coolant type and maintained at the proper concentration is sufficient for the heater circuit as well.