PACCAR MX-13 SPN 111 FMI 1 — Coolant Level Low

Code Details

Plain-English Meaning

The PACCAR MX-13 cooling system uses a sensor in the coolant reservoir to monitor whether the coolant level is above the minimum required for safe operation. When coolant drops below the sensor threshold, the ACM sets SPN 111 FMI 1 and the driver is notified through the instrument cluster. The MX-13 is a high-displacement six-cylinder engine used in heavy-duty linehaul and vocational applications that require the cooling system to reliably manage high heat loads over extended operation. Coolant loss that reduces the level below the sensor can quickly compromise the engine's thermal management under heavy load. External leaks from hose connections, the radiator, or the water pump are the most common causes. Internal leakage through a head gasket or aftertreatment cooling circuit is less common but must be investigated if no external leak is found. A failed sensor or wiring issue can also produce a false low reading.

SPN 111 is Coolant Level; FMI 1 indicates the sensor signal is valid and below the normal operational range. On the PACCAR MX-13, the coolant level sensor is positioned in the expansion or deaeration tank. PACCAR ESA provides the sensor live status and freeze-frame data at fault set. Physical verification of the actual coolant level is required alongside ESA diagnostics — ESA confirms sensor behavior, but the coolant level must be inspected directly to determine whether a real loss has occurred. If SPN 110 FMI 0 (high coolant temperature) is also stored, the coolant loss may already be impacting cooling system performance.

Common Symptoms

• Low coolant warning lamp or message on the instrument cluster
• Coolant reservoir visually low on inspection
• Possible external coolant leak visible at hose connections or the radiator area
• White exhaust smoke under load if coolant is entering the combustion chamber
• Rising coolant temperature if the cooling capacity is compromised

Possible Causes

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

• External coolant leak at radiator hoses, fittings, or connections
• Radiator core or tank failure from corrosion or damage
• Water pump seal failure
• Head gasket breach allowing coolant into the combustion chamber
• Aftertreatment cooling circuit leak if the MX-13 is equipped with aftertreatment cooling
• Coolant level sensor failure producing a false low reading

First Checks

• Check the coolant reservoir level with the engine cold — verify the level relative to the min/max marks on the reservoir
• Inspect all accessible hose connections, radiator end tanks, and the water pump for coolant seepage or wet residue
• Top up the coolant with the correct pre-mixed coolant to the proper level and monitor for further loss
• Connect PACCAR ESA to verify the coolant level sensor is reading correctly and check for related cooling system fault codes
• If no external leak is found, perform a combustion gas block test and check the engine oil for emulsification as indicators of internal coolant leakage

Can I Keep Driving?

Low coolant is a serious condition on the MX-13. The risk of overheating is significant under high-load operation with reduced coolant volume. Add coolant and investigate the source before resuming high-load linehaul operation.

Related Lookup Pages

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