On the 2015 Chevy Cruze with the 1.4L engine, the P0599 is a fairly straight forward code pointing to a problem with the Engine Coolant Thermostat Heater Control Circuit. On this vehicle, the thermostat heater is built into and part of the thermostat housing. Carefully inspect the electrical connector and wire harness for any loose, broken or damaged wires. If no problems are found, replace the thermostat housing (with built-in heater).
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