Cummins SPN 110 FMI 0 – Engine Coolant Temperature High Severity

Code Details

Plain-English Meaning

Every diesel engine has a temperature range it can safely operate in. When coolant temperature climbs above that range and reaches the high-severity level, the ECM activates engine protection. FMI 0 is the most serious above-normal indicator — the ECM is not just warning that things are warm, it is reporting an out-of-range condition that requires stopping the vehicle. Continuing to run a severely overheating engine risks head gasket failure, cylinder head warping, or internal damage that is far more expensive than the original cooling system fault.

SPN 110 is Engine Coolant Temperature 1 in J1939. FMI 0 means the measured value is valid and above the upper limit of the normal operational range at the highest severity level. On Cummins CM2350 ECMs, the coolant temperature sensor is a thermistor (NTC type) whose resistance decreases as temperature rises. A genuine overtemperature event — not a sensor failure — sets FMI 0. A failed sensor that shorts to supply voltage or opens would set FMI 3 or FMI 4 respectively. The ECM's engine protection module links coolant temperature to derate thresholds; severe overtemperature may also trigger a shutdown if configured.

Common Symptoms

• Red stop lamp or amber warning lamp active on the dash
• Engine protection derate — reduced available torque or a vehicle speed cap
• Coolant temperature gauge reading in the danger zone
• Possible steam from under the hood or coolant odor in the cab in severe cases
• Engine may enter a shutdown countdown if protection escalation is configured

Possible Causes

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

• Coolant loss from an external leak: failed hose, water pump seal, head gasket, or radiator fitting
• Thermostat stuck in the closed position, preventing hot coolant from reaching the radiator
• Fan clutch slipping or disengaged, reducing airflow through the radiator at lower road speeds
• Radiator core clogged with scale, debris, bugs, or insect buildup reducing heat rejection
• Water pump impeller slipping or failed, reducing coolant circulation rate
• Sustained high-load operation in high ambient temperatures exceeding the cooling system's capacity

First Checks

• Allow the engine to cool before opening the radiator or overflow tank — never open a hot pressurized system
• Check the coolant level in the overflow reservoir; a noticeably low level points directly to a leak
• Inspect visible hose connections, the water pump area, and the front of the radiator for coolant residue or wet spots
• Verify the fan engages and stays engaged when the engine is at operating temperature — a fan clutch that slips or cycles off too soon is a common cause in vocational duty cycles
• Connect Cummins Insite and review live coolant temperature alongside fan command output and any protection-event history

Can I Keep Driving?

Stop the vehicle safely when a red stop lamp is active or when this code is present alongside a significant derate. Running a seriously overheating engine risks terminal damage. Do not restart until the cause is identified — a short idle-cool period does not address the underlying fault.

Related Lookup Pages

• SPN 110 topic page
• FMI 0 explanation
• Cummins manufacturer hub
• Search this code