Cummins ISB 6.7 SPN 111 FMI 1 — Coolant Level Low

Code Details

Plain-English Meaning

The Cummins ISB 6.7 cooling system monitors engine coolant level through a sensor in the coolant expansion tank or overflow reservoir. When the coolant level drops below the minimum threshold, the ECM sets SPN 111 FMI 1 and the driver is notified. The ISB 6.7 is widely used in medium-duty trucks, school buses, and class 6-7 vocational vehicles where cooling system maintenance is critical because the engine frequently operates under high load at varying ambient temperatures. Coolant loss on the ISB 6.7 can result from external leaks at hose connections, the radiator, water pump seal wear, or internal leakage through a head gasket or EGR cooler failure. A failed coolant level sensor can also set this code without actual coolant loss. Physical verification of the coolant level is the essential first diagnostic step before further investigation.

SPN 111 is Coolant Level; FMI 1 indicates the signal is valid and below the normal operational range. On the Cummins ISB 6.7 (CM2350 B101), the coolant level sensor monitors the expansion tank level. Cummins INSITE provides access to the sensor live status and freeze-frame data for the fault event. The fault should always be accompanied by a physical coolant level check — INSITE data confirms sensor behavior, but the actual level must be verified by direct inspection. If the physical level is confirmed low, cooling system pressure testing locates external leaks. A combustion gas-in-coolant test identifies internal leakage. INSITE can also show whether SPN 110 FMI 0 (high coolant temperature) has been stored alongside SPN 111 FMI 1, which would suggest the coolant loss is significant enough to have already affected engine temperature.

Common Symptoms

• Low coolant warning lamp or instrument panel message
• Coolant reservoir visually low on inspection
• Possible coolant odor if leaking at hot surfaces near the exhaust
• White exhaust smoke if coolant is entering the combustion chamber
• Rising engine temperature gauge if coolant loss has reduced heat-transfer capacity

Possible Causes

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

• External coolant leak at a radiator hose, fitting, or clamp
• Radiator core or plastic tank leak from age, corrosion, or impact
• Water pump seal failure allowing coolant to weep externally
• Head gasket failure allowing coolant to enter the combustion chamber
• EGR cooler failure allowing coolant to enter the exhaust or oil system
• Coolant level sensor fault or wiring issue producing a false low reading

First Checks

• Check the coolant expansion tank level when the engine is cold — do not open a hot, pressurized cooling system
• Inspect all accessible hose connections, the radiator, and the water pump for visible coolant leakage or residue
• Add the correct pre-mixed coolant to restore the level and monitor the system for further loss during subsequent operation
• Connect Cummins INSITE and verify the coolant level sensor is reporting correctly — compare sensor status with the physical level to check for sensor discrepancy
• If no external leak is found, perform a block test for combustion gases in the coolant and check the engine oil for a milky or emulsified appearance that may indicate internal coolant entry

Can I Keep Driving?

Low coolant is a potentially serious condition. The ISB 6.7 may continue operating normally at low load if coolant loss is minor, but the risk of overheating increases rapidly under high load or in hot ambient conditions. Top up the coolant and investigate the source before high-load operation.

Related Lookup Pages

• SPN 111 topic page
• FMI 1 explanation
• Cummins manufacturer hub
• Search this code