Code Details
| Display code | SPN 3031 FMI 1 |
|---|---|
| SPN | 3031 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | PACCAR |
| 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 PACCAR MX-11 aftertreatment system uses a heated DEF tank to prevent diesel exhaust fluid from freezing in cold weather conditions. DEF freezes at about -11 degrees Celsius, and frozen DEF cannot be pumped from the tank to the dosing injector, which stops SCR function entirely. When the DEF tank temperature sensor reads below the normal range, the ACM sets SPN 3031 FMI 1 to alert the operator that DEF may be unavailable for dosing. On MX-11 trucks in Kenworth T370 and Peterbilt 337 medium-duty applications as well as Class 8 Kenworth and Peterbilt trucks, this fault is common during cold winter starts. The DEF tank heater uses engine coolant to warm the frozen DEF after a cold start, and normal dosing should resume once the DEF thaws. A failed heater or blocked coolant supply prevents this thaw and allows SCR efficiency faults to develop as DEF dosing remains unavailable.
SPN 3031 is Aftertreatment 1 DEF Tank Temperature; FMI 1 indicates valid data below the normal operational range. On the PACCAR MX-11, the DEF tank includes an integrated coolant-heated element that warms the DEF during cold weather operation. PACCAR ESA provides live DEF tank temperature data and related heater system status. Monitoring the DEF temperature during warmup is the primary method for confirming heater function — a temperature that rises progressively toward normal indicates the heater is working, while a static temperature tracking ambient after extended warmup indicates a heater circuit fault. The MX-11 is a downsized engine with a smaller DEF tank than the MX-13; the smaller tank can freeze more quickly in extreme cold and may require longer warmup before dosing resumes at cold extremes.
Common Symptoms
- DEF system or SCR warning lamp on the instrument cluster in cold weather
- SCR efficiency or DEF dosing fault codes appearing alongside SPN 3031 FMI 1
- Engine derate if DEF dosing is interrupted long enough for inducement to advance
- DEF pump inactivity 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 cold soak in 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
- Assess overnight ambient temperature and determine if below-freezing conditions occurred
- Allow extended engine warmup and monitor DEF tank temperature live in PACCAR ESA
- If the temperature is not rising during warmup, inspect the coolant supply lines to the DEF tank heater for blockage or disconnection
- Confirm the coolant supply lines at the DEF tank heater are warm during engine operation as a basic circuit check
- Test the DEF temperature sensor reading against known ambient temperature to assess plausibility
Can I Keep Driving?
The MX-11 is driveable with SPN 3031 FMI 1 active. DEF dosing resumes once the DEF thaws. If the heater has failed, the DEF will remain frozen until ambient temperatures rise, and SCR efficiency faults will develop.
Related Codes
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
Does the PACCAR MX-11 DEF tank heater use engine coolant or an electric element?
The PACCAR MX-11 DEF tank heater uses engine coolant as the heat source. Hot engine coolant circulates through a heat exchanger integrated into the DEF tank structure, transferring heat to the DEF. Some DEF supply line sections also use coolant-heated lines to prevent the DEF from refreezing after leaving the tank. An electric pre-heat element may supplement the coolant heater on some configurations to accelerate thaw time at extreme cold temperatures, but the primary heater is coolant-based.
Can SPN 3031 FMI 1 on the MX-11 appear in summer from a sensor fault?
Yes. In warm ambient conditions where DEF freezing is not possible, SPN 3031 FMI 1 is most likely caused by a DEF tank temperature sensor that has failed and is producing a below-normal reading. Comparing the sensor's reading in PACCAR ESA against the actual ambient temperature and engine coolant temperature can reveal an unrealistic reading — for example, a sensor showing -20 degrees C when the ambient temperature is 25 degrees C. Sensor replacement is the appropriate repair when a false reading in warm conditions is confirmed.
Will driving for a while automatically resolve SPN 3031 FMI 1 on the MX-11 in winter?
If the DEF tank heater is functioning properly, extended engine operation will eventually thaw the frozen DEF through the coolant heater, and SPN 3031 FMI 1 will become inactive once the DEF temperature returns to the normal range. The time required depends on how thoroughly the DEF is frozen and the ambient temperature. Driving at normal operating conditions where the engine coolant reaches full operating temperature is more effective at thawing the DEF than extended idling at low coolant temperatures. If the fault persists after 45 to 60 minutes of normal operation, the heater system should be investigated.