P0597
PowertrainThermostat Heater Control Circuit Open
The thermostat heater control circuit is open or faulty. Some modern engines have an electrically-heated thermostat that the ECU can command to open earlier than the coolant temperature would normally trigger, this gives more precise control of warm-up and emissions.
ⓘ Information only. This page provides general educational information about fault code P0597. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0597 mean?
P0597 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Thermostat Heater Control Circuit Open.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light
- • Slightly worse fuel economy on motorway runs (the engine may run cooler than optimal)
- • Coolant temperature reading possibly slightly different from normal
- • Generally drivability is unaffected
- • On some cars, the cooling fans may run more often
Possible causes
Causes commonly associated with P0597, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed thermostat heater element (the heater inside the thermostat has gone open-circuit, most common at 80,000+ miles)
- 2. Wiring damage between thermostat and ECU, often from heat exposure in the engine bay
- 3. Connector at the thermostat corroded or unplugged, the location is exposed to road grime
- 4. Blown fuse in the heater circuit
- 5. Failed ECU output stage (last resort, rare)
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Visual inspect the thermostat connector for damage or corrosion
- 2. Check the relevant fuse for the heater circuit
- 3. Resistance-test the heater element across the thermostat's heater pins, a healthy heater is typically a few ohms, an open circuit reads infinite
- 4. Verify 12V supply at the heater terminal with the ignition on
- 5. Check ground continuity back to the ECU
Common questions about P0597
What does an electronically-heated thermostat actually do? +
Conventional thermostats open at a fixed coolant temperature, around 88-92°C typically. An electronically-heated unit lets the ECU force the thermostat open earlier under specific conditions, for instance during high-load driving when keeping the engine slightly cooler reduces emissions and protects the cat. The heater is the small electrical element built into the thermostat housing that the ECU can energise on demand.
Is this serious? +
Not particularly. The thermostat still works as a normal mechanical thermostat even with the heater failed, the ECU just loses its ability to force early opening. Drivability is fine, fuel economy might be marginally worse on long runs. Worth fixing for the warning light rather than for any serious mechanical reason.
Will it pass MOT? +
Code itself doesn't directly fail MOT, but if the engine warning light is on at the time of test, that's an automatic fail under post-2018 rules. Fix the underlying fault, drive a few cycles to clear the light, then book the test.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →