What does it mean?
When a malfunction indicator lamp is accompanied by a trouble code P0599, it means that the powertrain control module (PCM) has detected a malfunction (high or excessive voltage) in the thermostat heater control circuit of the engine cooling system. Only vehicles equipped with an electronically heated thermostat should exhibit a code P0599. In this case, the malfunction may be electrical or mechanical.
Typically, engine coolant temperature is monitored (by the PCM) with one or more engine coolant temperature sensors (ECT). The ECT sensor/s consist of a brass (or plastic) housing which has a thermal resistor inside. The ECT sensor housing is designed so that it may be threaded into a coolant passage of the engine block, cylinder head, or intake manifold. When the thermostat opens, coolant flows across the tip of the ECT sensor (where the thermal resistor is).
The ECT sensor is provided a reference voltage (usually 5-volts) and a ground. The sensor completes the circuit. As engine coolant temperature increases, the resistance of the thermal resistor decreases. The result of this decrease in circuit resistance is higher voltage being input to the PCM. When engine coolant temperature decreases, a reverse action occurs and circuit voltage decreases. The PCM interprets these variations in circuit voltage as changes in engine coolant temperature.
The spring-loaded thermostat operates by opening when engine coolant reaches a certain temperature (usually 180-degrees F). A sealed capillary is filled with wax and a rod is inserted into the wax. The rod is then attached to a plunger in the thermostat. When the wax becomes hot, it expands inside the tightly sealed capillary; pushing the rod upward and opening the plunger. With the plunger open, coolant may flow freely between the engine and radiator.
The electronically heated thermostat uses an integrated heater element to increase capillary pressure in a more controlled manner. The heater element increases capillary temperature much faster than engine coolant. This allows the PCM to control thermostat operation regardless of coolant temperature or coolant level. Every automobile utilizes some type of thermostat to regulate engine coolant flow and maximize performance.
The electronically controlled thermostat simply gives the PCM an added level of control over the flow of engine coolant in certain situations. Thermostat heater control voltage is constantly monitored by the PCM during engine operation. If the PCM detects a degree of thermostat heater control voltage that exceeds the maximum threshold, a code P0599 will be stored and a malfunction indicator lamp (MIL) may be illuminated.
Multiple ignition cycles (with a failure) may be required for MIL illumination.