PACCAR MX-11 SPN 111 FMI 1 — Coolant Level Low

Code Details

Plain-English Meaning

The PACCAR MX-11 cooling system uses a coolant level sensor in the expansion reservoir to monitor coolant volume above the minimum required level. When coolant drops below the sensor threshold, the ACM sets SPN 111 FMI 1 and notifies the operator through the instrument cluster. The MX-11 is a downsized high-output diesel used in Kenworth and Peterbilt Class 8 trucks that require reliable thermal management under demanding duty cycles. Coolant loss on the MX-11 can originate from external hose leaks, the radiator, the water pump seal, or less commonly from internal sources such as head gasket failure or a failed aftertreatment cooling circuit component. A failed coolant level sensor or wiring fault can produce a false low-level reading, so physical coolant level verification is the necessary first step before further diagnostic work is performed.

SPN 111 is Coolant Level; FMI 1 indicates valid sensor data below the normal operational range. On the PACCAR MX-11, the coolant level sensor monitors the expansion tank level and communicates its state to the ACM. PACCAR ESA provides the sensor live status and freeze-frame data at fault detection. Physical coolant level inspection is required alongside ESA diagnostics to confirm whether an actual level deficiency exists. If SPN 110 FMI 0 (high coolant temperature) is also stored, the coolant loss is already affecting thermal management and the situation is more urgent. Cooling system pressure testing is used to identify external leaks, and a combustion block test identifies internal leakage through the head gasket or cooling circuit components.

Common Symptoms

• Low coolant warning lamp or message on the instrument panel
• Coolant reservoir visually low on physical inspection
• Visible coolant leakage at hose connections or the radiator area
• White exhaust smoke if coolant is entering the combustion chamber
• Engine temperature rising if the cooling system capacity is compromised

Possible Causes

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

• External coolant leak at hose connections, fittings, or clamps
• Radiator core or tank failure from corrosion, age, or impact
• Water pump shaft seal failure
• Head gasket breach allowing coolant into the combustion chamber
• Coolant level sensor failure producing a false low-level reading

First Checks

• Inspect the coolant reservoir level with the engine cold and compare to the reservoir min/max marks
• Check all accessible hose connections, the radiator, and the water pump area for visible coolant leakage
• Top up the coolant to the correct level with the specified coolant mixture and monitor the level over subsequent operation
• Connect PACCAR ESA to verify the coolant level sensor is reading correctly and check for cooling system related fault codes
• If no external source is identified and the coolant continues to decrease, perform a combustion block test and inspect the engine oil for emulsification indicating internal leakage

Can I Keep Driving?

Low coolant is a serious condition on the MX-11. Avoid high-load operation with confirmed low coolant. Top up and identify the source before resuming heavy-duty service.

Related Lookup Pages

• SPN 111 topic page
• FMI 1 explanation
• Search this code