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 DD15 is a high-capacity linehaul engine that relies on the DEF system for emissions compliance. The DEF tank heater — a coolant-heated element integrated into the DEF tank — maintains 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 range, the ACM sets SPN 3031 FMI 1 to alert the driver. If the DEF is frozen, the DEF supply pump cannot draw fluid from the tank, disrupting DEF dosing and leading to SCR efficiency faults. On linehaul DD15 trucks that are parked outside overnight in winter climates, this fault is a common cold-start event that resolves once the engine warmup thaws the DEF through the tank heater. A persistent or recurrent fault after extended warmup indicates a heater system problem.
SPN 3031 is Aftertreatment 1 DEF Tank Temperature; FMI 1 indicates valid data below the normal range. On GHG17 DD15 engines, the DEF tank heater uses engine coolant to maintain DEF temperature. Detroit DiagnosticLink provides live DEF tank temperature readings and related DEF system status data. The temperature trend during warmup — rising progressively or remaining flat — is the key diagnostic indicator of heater function. A temperature that rises during warmup confirms the heater is transferring heat to the DEF. A static temperature that tracks ambient even after 30 or more minutes of engine operation suggests a heater coolant supply fault.
Common Symptoms
- DEF system warning lamp in cold weather start conditions
- SCR efficiency or DEF dosing faults developing alongside SPN 3031 FMI 1
- Engine derate if DEF dosing remains interrupted and SCR inducement advances
- Cold-start DEF pump inactivity 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 in temperatures below -11 degrees C
- DEF tank heater coolant supply failure — blocked line, failed thermostatic valve, or disconnected hose
- DEF tank heater element failure
- DEF tank temperature sensor failure producing a false below-normal reading
First Checks
- Assess overnight ambient temperature and parking conditions
- Allow extended engine warmup and monitor DEF tank temperature via DiagnosticLink
- If temperature is not rising during warmup, inspect the coolant supply lines to the DEF tank heater for blockage or disconnection
- Check the DEF heater coolant flow by confirming the coolant lines at the DEF tank connection are warm during engine operation
- Test the DEF temperature sensor plausibility against ambient and coolant temperature readings
Can I Keep Driving?
The DD15 is driveable with SPN 3031 FMI 1 active. DEF dosing will resume once the DEF thaws. A malfunctioning heater requires repair to prevent recurring SCR system disruption in cold weather.
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
Does the DD15 DEF tank heater turn off once the DEF reaches operating temperature?
Yes. The DEF tank heater coolant supply is typically controlled by a thermostatic valve or the DEF system temperature management calibration that reduces or stops coolant flow to the heater once the DEF temperature is above the target level. This prevents overheating of the DEF, which can degrade urea quality at sustained high temperatures. The heater activates again if the DEF temperature drops back toward the lower threshold during cold ambient conditions.
Can SPN 3031 FMI 1 on the DD15 occur in summer if the heater is stuck open?
A DEF tank heater thermostatic valve that is stuck open will allow engine coolant to flow to the heater even when DEF warming is not needed. This can cause the DEF to overheat in warm ambient conditions, degrading the urea quality. An overheating DEF condition typically sets a high-temperature fault rather than the below-normal SPN 3031 FMI 1. SPN 3031 FMI 1 in warm weather is more likely a sensor fault than a heater-stuck-open condition.
How do I prevent SPN 3031 FMI 1 on the DD15 in cold weather fleet operations?
The most effective prevention for SPN 3031 FMI 1 in cold weather is ensuring the DEF tank heater system is inspected and serviced at regular maintenance intervals — checking coolant supply line condition, heater element function, and thermostatic valve operation. Pre-warming the truck in a heated bay before a cold-weather start allows the engine to reach operating temperature before going into service, which reduces the warmup time required to thaw the DEF. For fleets in extreme cold climates, DEF anti-freeze additives are not permitted per ISO 22241, so heater reliability is the primary defense against DEF freezing.