Faulty electronically controlled thermostat with a failed internal heater element; Damaged, corroded or loose connector at the thermostat housing; Open circuit or chafed wiring between the thermostat and the control unit, often near hot components; Blown fuse or faulty relay supplying the thermostat heater, where fitted; Fault in the engine control unit output driver; Water ingress or coolant leak contaminating the thermostat connector.

P0597

Thermostat Heater Control Circuit/Open.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
97: FaultThe specific fault in that area

What it means

P0597 is the generic code for an open circuit in the thermostat heater control circuit. The engine control unit drives a heating element in an electronically controlled thermostat so the engine can run hotter or cooler depending on load. It sets the code when it sees no current or a voltage that shows the circuit is open when it commands the heater on or off.

Symptoms

Engine warning light on
Engine running cooler than normal or taking a long time to warm up
Poor cabin heater output in cold weather
Temperature gauge reading low or fluctuating
Slightly higher fuel consumption
Cooling fan running more than usual

Possible causes

Faulty electronically controlled thermostat with a failed internal heater elementlikely
Damaged, corroded or loose connector at the thermostat housinglikely
Open circuit or chafed wiring between the thermostat and the control unit, often near hot componentspossible
Blown fuse or faulty relay supplying the thermostat heater, where fittedpossible
Fault in the engine control unit output driverrare
Water ingress or coolant leak contaminating the thermostat connectorrare

How to diagnose

Read all stored codes and freeze frame data, noting engine temperature, load and whether other cooling or temperature codes are present.
Check coolant level and look for leaks around the thermostat housing.
Inspect the thermostat connector and wiring for corrosion, heat damage, chafing, loose pins or coolant contamination.
Check the relevant fuse and relay if the circuit uses them, as per the wiring diagram for the model.
With a scan tool that supports it, watch coolant temperature and command the thermostat heater to see whether it responds.
Measure the heater element resistance at the thermostat and compare it with the manufacturer specification for that engine.
Check the supply, earth and control signal at the connector with the ignition on, following the wiring diagram.
Test the wiring for continuity back to the control unit, and only suspect the control unit once everything else is proven good.

How to fix

Replace the electronically controlled thermostat assembly if the heater element is open circuit.
Repair or replace damaged connectors and terminals at the thermostat.
Repair or replace open or chafed wiring in the heater circuit.
Replace a blown fuse or faulty relay after finding out why it failed.
Top up or refill coolant and bleed the system correctly after any thermostat work.
Repair or replace the engine control unit only if a driver fault is confirmed.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, as the thermostat will normally still open and close mechanically, but fix it soon. Watch the temperature gauge, and stop if the engine overheats or the coolant level drops.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.