The thermostat heater has the function of controlling the coolant flow, as well as regulating the engine temperature while the engine is running. The thermostat is supplied with 12 volts of power by the ECM (Engine Control Module).
When the engine thermostat system exhibits erratic operation, a series of fault codes can be set to indicate such a condition to the driver. In the case of the DTC P0598 OBDII code, it indicates that the voltage in the thermostat control circuit is well below the allowable voltage, causing unstable system operation.