P0597 Thermostat Heater Control Circuit/Open

P0597 indicates an open thermostat heater control circuit. The controller does not see expected continuity or current in the electrically controlled thermostat heater path.

Code details

Generic OBD-II · P0

System: Speed / Idle
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace coolant temperature sensors before testing the thermostat heater circuit and actual warm-up behavior.

Symptoms of P0597

Unstable, high, or low idle, or stalling.
Erratic speedometer, cruise-control, or shift behaviour.
Check-engine light with a hesitation or surge.

Common causes of P0597

A dirty throttle body, idle-control valve, or throttle-position sensor.
A failing vehicle-speed or wheel-speed sensor, or its wiring.
Vacuum leaks or a related sensor forcing the idle/speed strategy.

Useful checks before replacing parts

Check thermostat heater command, supply voltage, control continuity, heater resistance, coolant temperature, warm-up behavior, and freeze-frame data.
Inspect thermostat connector, wiring opens, fuse feed, ground or driver path, coolant intrusion, damaged terminals, and recent thermostat replacement work.
Command the heater where supported and verify both electrical current and thermostat temperature response.

Related OBD-II codes

These nearby speed / idle entries can help compare wording and pin down the fault area.