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 ISB 6.7 DEF system includes a heated tank to prevent the DEF from freezing in cold weather operation. When the DEF tank temperature drops below the normal operational range — as indicated by SPN 3031 FMI 1 — the ECM alerts the operator because DEF that is too cold may be frozen and unavailable for dosing. For medium-duty applications including school buses and delivery trucks, the ISB 6.7 may be parked outdoors in cold climates where overnight freezing is common. The DEF tank heater is designed to thaw the DEF after a cold start using engine coolant heat. If the heater is functioning, the DEF temperature will rise toward normal during engine warmup. If the heater has failed, the DEF temperature will remain near ambient even after extended engine operation, and DEF dosing will remain interrupted until the DEF thaws from ambient temperature warming alone.
SPN 3031 is Aftertreatment 1 DEF Tank Temperature; FMI 1 indicates valid data below the normal operational range. On the Cummins ISB 6.7 (CM2350 B101), the DEF tank includes a temperature sensor and a heater circuit. Cummins INSITE provides live DEF tank temperature data and heater status information. Monitoring the DEF temperature during engine warmup in cold conditions confirms whether the heater is effectively warming the DEF — the temperature should rise progressively as the heater transfers heat to the DEF. A heater that is not functioning will show a DEF temperature that tracks ambient temperature rather than rising during engine warmup.
Common Symptoms
- DEF or SCR system warning lamp on the instrument cluster in cold weather
- SCR efficiency fault codes developing alongside SPN 3031 FMI 1 if DEF dosing is interrupted
- Engine derate if DEF dosing remains unavailable and inducement advances
- No DEF pump activity during cold start-up until the DEF thaws
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF frozen from overnight exposure to ambient temperatures below -11 degrees C
- DEF tank heater failure — coolant supply line clogged, heater element failed, or electrical circuit fault
- DEF tank temperature sensor failed producing a false low reading
First Checks
- Check ambient temperature and determine if overnight freezing conditions occurred
- Allow additional warmup time in cold weather before expecting DEF dosing to resume
- Connect Cummins INSITE and monitor DEF tank temperature live data during engine warmup — temperature rising toward normal confirms the heater is working
- If temperature does not rise during warmup, inspect the coolant supply circuit to the DEF tank heater for blockage or a failed heater element
- Verify the DEF temperature sensor reading is plausible relative to known conditions
Can I Keep Driving?
The ISB 6.7 is driveable with SPN 3031 FMI 1 active. DEF dosing is interrupted while DEF is frozen. Allow adequate warmup before expecting SCR efficiency faults to clear.
Related Codes
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
Can I add warm DEF to the ISB 6.7 tank to help thaw frozen DEF?
Adding a small amount of room-temperature or warm DEF to the tank when the level is low and the existing DEF is frozen can help introduce unfrozen fluid near the pump pickup, potentially allowing some dosing to resume sooner. However, this is only practical if the tank level allows additional fluid to be added safely. The DEF tank heater is the proper mechanism for thawing the bulk of the frozen DEF. If the heater is not functioning, the root cause of the heater failure should be diagnosed and repaired rather than relying on ambient warming alone.
Does the DEF freeze and thaw cycle damage the DEF quality on the ISB 6.7?
DEF is designed to withstand multiple freeze-thaw cycles without degradation when it is ISO 22241-compliant quality DEF. The freezing and thawing process does not change the urea concentration or contaminate the DEF. After thawing, the DEF returns to its normal liquid state and resumes normal dosing effectiveness. The main concern with repeated freeze-thaw cycles is ensuring the DEF tank and supply system hardware remain undamaged from thermal expansion stress.
Will SPN 3031 FMI 1 on the ISB 6.7 go away on its own after the engine warms up?
If the DEF heater is functioning, the DEF tank temperature will rise above the fault threshold during engine warmup and SPN 3031 FMI 1 will become inactive once the temperature returns to the normal range. The stored fault code will remain in INSITE history until manually cleared, but the active fault and any associated SCR efficiency faults should clear as the DEF warms and dosing resumes. If the fault persists after an extended warmup period, the DEF heater system should be investigated.