What does P0597 thermostat heater control circuit open mean?
Short answer: Some engines use an electrically heated thermostat to adjust operating temperature, and P0597 means the computer found an open circuit in the heater, its wiring or connector.
An electrically heated thermostat lets the engine computer change operating temperature. Under light load the engine runs warmer for efficiency; under heavy load the computer heats the thermostat element so it opens earlier and the engine runs cooler. When the heater circuit is open, the computer sets P0597, and the engine may run at a fixed temperature.
Check the connector at the thermostat housing first, since coolant seepage and heat often corrode the pins. Measure the heater resistance through the connector and compare it with the specification. Then check the power feed and the computer's control signal, using a scan tool command if available.
The heater is usually part of the thermostat assembly, so replacement means draining coolant and installing the complete unit with a new seal. Let the engine cool before opening the cooling system to avoid scalding coolant, and bleed air from the system afterward using the maker's procedure or a vacuum fill tool.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.