Failed thermostat heater element inside the thermostat housing; Broken or corroded wire in the thermostat heater circuit; Loose or damaged electrical connector at the thermostat housing; Open fuse or relay supplying power to the thermostat heater circuit; Faulty engine control module (ECM) driver for the thermostat heater, though uncommon; Corrosion or water intrusion in the wiring harness near the thermostat

DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P0597

P0597Thermostat Heater Control Circuit/Open

Q: What does code P0597 mean?

Thermostat Heater Control Circuit/Open. P0597 means the engine computer has detected an open circuit or abnormal resistance in the electrical heater element built into the engine thermostat housing. The computer commands the thermostat heater on to speed up engine warm-up, but the circuit does not respond as expected, so the check engine light is set.

P0597 means the engine computer has detected an open circuit or abnormal resistance in the electrical heater element built into the engine thermostat housing. The computer commands the thermostat heater on to speed up engine warm-up, but the circuit does not respond as expected, so the check engine light is set.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

Check engine light illuminated, often with no other noticeable symptoms
Engine may take longer than normal to reach operating temperature
Coolant temperature gauge may stay low or rise very slowly
Heater performance inside the cabin may be weak during cold starts
Possible slight decrease in fuel economy due to extended cold-engine operation
In rare cases, the engine may run slightly rough or idle high while warming up

Common causes, most likely first

Failed thermostat heater element inside the thermostat housing
Broken or corroded wire in the thermostat heater circuit
Loose or damaged electrical connector at the thermostat housing
Open fuse or relay supplying power to the thermostat heater circuit
Faulty engine control module (ECM) driver for the thermostat heater, though uncommon
Corrosion or water intrusion in the wiring harness near the thermostat

How to diagnose and fix it

Scan all modules and record freeze-frame data to confirm P0597 is the only related code
Visually inspect the thermostat heater connector and wiring for damage, corrosion, or looseness
Check the thermostat heater circuit fuse and relay, and verify power and ground at the connector
Measure the resistance of the thermostat heater element and compare to the manufacturer's specification
Perform a voltage drop test on the ground and power sides of the circuit while commanding the heater on with a scan tool
If the circuit tests good, replace the thermostat assembly with the integrated heater
Clear codes and verify the repair by running the engine and confirming the heater circuit operates correctly

Why a thermostat heater code is not always the thermostat

A P0597 can be set by a simple blown fuse or a corroded connector pin, so always check the cheap electrical items before replacing the thermostat assembly.
Some vehicles use the thermostat heater to control coolant flow for emissions and fuel economy, so a fault may not cause any driveability complaint at all.
The thermostat heater circuit is often powered through a relay that can fail intermittently, causing the code to set only on cold starts or in damp weather.
A scan tool that can command the thermostat heater on and off is essential for testing the circuit under load; a simple ohmmeter check can miss an intermittent open.
Do not overlook the engine coolant temperature sensor circuit, because a skewed sensor reading can make the computer command the heater incorrectly and set related codes.

Can I keep driving with P0597?

The vehicle is safe to drive with P0597 because the thermostat still regulates coolant temperature mechanically, but avoid extended idling and short trips until the fault is repaired. Prolonged driving with a cold-running engine can reduce fuel economy and increase emissions, so schedule diagnosis soon.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.