P0597: Thermostat Heater Control Circuit/Open

A map-controlled thermostat is a wax thermostat with a built-in heating resistor (Valeo: about 10 Ω). Unheated it opens late, for hot running at part load; the engine module heats it by map under full load or towing so it opens earlier. CARB requires the thermostat to be monitored and the heater circuit counts as an output component. GM switches it to ground with a low-side driver and checks it every second, fault after 15 of 30 samples, type B; Hyundai/Kia/Genesis and VW/Audi use the codes as well. P0597 means the heater circuit is open – GM above 200 kΩ with the driver off. The thermostat then only works through its wax and opens late, so coolant runs hotter under load; Audi's 4.2 FSI also limits engine speed. Genesis (G80/G90 3.3 T, 3.8) replaced thermostats from an early production lot.

Symptoms

• Check engine light on (after two trips)
• Coolant temperature higher than usual under load
• Audi/VW: coolant warning and engine speed limit possible

Most likely causes

• Heating resistor in the thermostat open
• Connector at the thermostat loose or corroded, coolant in the connector
• Broken wire or missing supply (fuse, relay)
• Engine module driver

What to check, in order

Start with the simplest. You can tick off each step.

1. With the engine cold, check the coolant level; never do this when the system is hot
2. Check the continuity of the wire between the component and the control module.
3. Check the actual temperature in live data
4. Make sure the fan comes on when it should
5. Check the thermostat and connector
6. Measure the sensor and replace it if its readings do not make sense

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P0597 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 5: Speed and idle
• 97: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0597?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P0597?

The most common causes are: heating resistor in the thermostat open; connector at the thermostat loose or corroded, coolant in the connector; broken wire or missing supply (fuse, relay).

Related codes

• P0598Thermostat Heater Control Circuit Low
• P0599Thermostat Heater Control Circuit High
• P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
• P2181Cooling System Performance
• P0217Engine Coolant Over Temperature Condition

Sources

• gsi.ext.gm.com: 18OBDG03A Engine Diagnostics
• static.nhtsa.gov: MC 10183956 0001
• static.nhtsa.gov: MC 10191796 0001
• static.nhtsa.gov: MC 10128349 9999

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.