Code Details
| Display code | SPN 3031 FMI 1 |
|---|---|
| SPN | 3031 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | Detroit Diesel |
| 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 Detroit DD13 aftertreatment system includes a DEF tank heater that uses engine coolant to warm the DEF during cold weather operation. The DEF tank temperature sensor monitors the DEF temperature and reports it to the ACM. When the DEF is at or below freezing temperature, the ACM sets SPN 3031 FMI 1 to alert the driver that DEF may be unavailable for dosing. On GHG17 DD13 engines, the DEF system is designed to resume normal operation after the tank heater thaws the DEF following a cold start. If the heater fails, the DEF remains frozen and DEF dosing cannot occur, which leads to SCR efficiency faults and eventual inducement derate 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 GHG17 DD13 engines, the DEF tank assembly includes an integrated heater and temperature sensor. Detroit DiagnosticLink provides live DEF tank temperature data and heater circuit status. Monitoring the DEF temperature rise during engine warmup in cold conditions confirms heater function. A properly functioning heater shows the temperature rising progressively from the cold-start ambient reading toward the normal operational range. A failed heater shows the temperature remaining near the ambient reading even after extended engine operation.
Common Symptoms
- DEF temperature or SCR system warning lamp on the instrument cluster in cold weather
- SCR efficiency or DEF dosing fault codes alongside SPN 3031 FMI 1
- Engine derate if DEF dosing remains interrupted and inducement advances
- Cold-start DEF pump silence until DEF thaws
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Frozen DEF from overnight cold soak in temperatures below -11 degrees C
- DEF tank heater failure — coolant supply blockage or heater element failure
- DEF tank temperature sensor failure producing a false below-normal reading
First Checks
- Assess ambient temperature history — was the truck parked in freezing conditions?
- Allow extended engine warmup time and connect DiagnosticLink to monitor DEF tank temperature rise
- If temperature does not rise during warmup, check the DEF tank heater coolant supply circuit for blockage or a disconnected coolant line
- Test the DEF temperature sensor plausibility against the known ambient temperature
- Check for SCR efficiency-related codes in DiagnosticLink that confirm DEF dosing was interrupted
Can I Keep Driving?
The DD13 is driveable with SPN 3031 FMI 1. DEF dosing is suspended until the DEF thaws. If the heater has failed and the DEF remains frozen, SCR efficiency faults will develop.
Related Codes
Related Lookup Pages
Sources
- Detroit Diesel Service and Diagnostic Resources — Public Reference Detroit Diesel (Daimler Truck North America) · oem · accessed 2026-06-11 · confidence medium
Source: Detroit Diesel (Daimler Truck North America), Detroit Diesel Service and Diagnostic Resources — Public Reference. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - DiagnosticLink Detroit Diesel Corporation · oem · accessed 2026-05-05 · confidence medium
Source: Detroit Diesel Corporation, DiagnosticLink. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
How does the DD13 DEF tank heater work?
The DD13 DEF tank heater is a coolant-heated heat exchanger integrated into the DEF tank assembly. Engine coolant from the engine cooling circuit flows through the heater element when the engine is running, transferring heat to the DEF in the tank. The coolant supply to the heater is typically controlled by a thermostatic valve that allows coolant to flow to the heater when the DEF temperature is below a certain threshold. If the coolant supply line to the heater becomes blocked — from calcium deposits, a kinked hose, or a failed thermostatic valve — coolant will not reach the heater and the DEF will not warm.
Can SPN 3031 FMI 1 on the DD13 set in temperatures above freezing?
SPN 3031 FMI 1 can set in above-freezing ambient conditions if the DEF tank temperature sensor has failed and is producing a false below-normal reading. If the fault appears during warm weather when actual DEF freezing is not plausible, the sensor and its wiring circuit should be evaluated for a fault. DiagnosticLink live data can be used to compare the DEF tank temperature reading against the actual ambient temperature and the engine coolant temperature to determine whether the reading is plausible.
Will SPN 3031 FMI 1 on the DD13 clear without any action once the weather warms?
If the fault is caused by frozen DEF from a cold ambient soak and the DEF heater is functioning, the DEF will thaw during engine warmup and SPN 3031 FMI 1 will become inactive when the DEF tank temperature returns to the normal operational range. As ambient temperatures rise above the DEF freezing point, the fault is less likely to recur without heater intervention. However, the stored fault code in DiagnosticLink will remain until manually cleared.