Voltage from the engine control module (ECM) is applied to the engine coolant level (ECL) switch when the ignition switch is in the ON position. When the reservoir runs low, the float will be lowered on the coolant level sensor. The signal to the ECM will indicate low coolant.
If the engine control module (ECM) detects that the coolant level switch has experienced a power loss or that the level can not be detected, the ECM will set a diagnostic trouble code (DTC) and derate the engine to avoid engine damage.

Conditions for diagnostic trouble code (DTC):
1) Low coolant level

Probable cause
1) Low coolant level
2) Engine Coolant Level (ECL) Sensor
3) Connector
4) Wiring harness
5) Engine control module (ECM)