Cummins ISX15 / X15 SPN 3031 FMI 1 — DEF Tank Temperature Below Normal

Code Details

Plain-English Meaning

Diesel exhaust fluid freezes at approximately -11 degrees Celsius (12 degrees Fahrenheit), which occurs in cold weather climates or when a truck is parked outside overnight in winter conditions. The Cummins ISX15 and X15 aftertreatment system includes a DEF tank heater and supply line heater that warm the DEF to keep it fluid and pumpable during cold-weather operation. The DEF tank temperature sensor monitors the temperature of the DEF in the tank. When SPN 3031 FMI 1 is set, the sensor is reading below the normal operational range — either the DEF has frozen and the heater has not yet warmed it, or the tank heater has failed and is not warming the DEF as it should during cold operation. If the DEF is frozen, the DEF pump cannot deliver DEF to the dosing injector, resulting in zero DEF injection and rapidly declining SCR efficiency. The heater system is designed to thaw the DEF within a reasonable warmup period after engine start.

SPN 3031 is Aftertreatment 1 DEF Tank Temperature; FMI 1 indicates the sensor signal is valid and below the normal operational range. On CM2350 ISX15 and X15 engines, the DEF tank heater is integrated into the DEF tank assembly and uses engine coolant or electric heating to maintain the DEF above its freezing point during cold operation. Cummins INSITE provides live DEF tank temperature data and DEF heater system status. If the temperature reading indicates DEF is frozen, INSITE may show SCR efficiency faults developing simultaneously as DEF dosing is interrupted. A tank heater that has failed to operate will typically be indicated by the DEF temperature remaining near ambient temperature even after extended engine operation in cold conditions, when the DEF should have thawed if the heater were functioning.

Common Symptoms

• DEF tank temperature warning on the instrument cluster in cold weather
• SCR efficiency faults or DEF system warnings appearing alongside SPN 3031 FMI 1
• Engine derate if DEF is unable to be delivered and SCR efficiency fault inducement advances
• No DEF pump activity or DEF dosing during cold start-up until the DEF thaws

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

• Ambient temperature below the DEF freezing point causing the DEF to freeze overnight or during extended cold soak
• DEF tank heater failure — heater element, coolant supply, or electric heating circuit fault
• DEF tank temperature sensor failure producing a false low-temperature reading
• DEF supply line not fully heated during extreme cold conditions

First Checks

• Determine the ambient temperature and operating history — has the truck been parked in freezing conditions, or has the DEF temperature remained low during operation?
• Allow additional engine warmup time in cold conditions — the DEF tank heater system requires time to thaw frozen DEF after a cold start
• Connect Cummins INSITE to monitor DEF tank temperature live data and confirm whether the temperature is rising during engine warmup, which would indicate the heater is functioning
• If the temperature does not rise during extended warmup, check the DEF tank heater circuit — inspect the coolant supply lines to the heater (if coolant-heated) or the electrical connection (if electrically heated)
• Test the DEF tank temperature sensor reading against ambient temperature for plausibility when the DEF should be fully thawed after extended warmup

Can I Keep Driving?

The engine is typically driveable with SPN 3031 FMI 1. DEF dosing will be interrupted until the DEF thaws. If DEF dosing remains interrupted, SCR efficiency faults will develop and the inducement sequence will begin.

Related Lookup Pages

• SPN 3031 topic page
• FMI 1 explanation
• Cummins manufacturer hub
• Aftertreatment fault codes
• Search this code