Trouble Code P0597: Thermostat Heater Control Circuit Open

P0597 is a diagnostic trouble code that can be thrown by any OBD-II equipped vehicle, which includes every model sold in the United States since 1996. If you plug your scanner in and get this trouble code, you are likely wondering: what does the code P0597 mean? And, how can I fix it?

What Does the Code P0597 Mean?

Code P0597, as well as the related codes P0598 and P0599, refers to an electronically-controlled thermostat in the engine. Although these codes are generic and can be set by any OBD-II equipped vehicle, only ones with this type of thermostat will throw it. Many manufacturers don’t use this type of system.

This error is most common on German manufactured-vehicles such as the brands Audi, BMW, Mercedes-Benz, Mini, Opel and Volkswagen. The component includes temperature sensors, a control housing, a thermostat and a computer program.

The thermostat helps with managing the engine temperature and controlling the flow of coolant. The code is set when the PCM or ECM detects that the system is experiencing a problem with control voltage, most likely either an open or short circuit.

What Are the Symptoms of P0597?

There are typically not many noticeable symptoms of this issue. The primary indicator is that the check engine light will illuminate. Depending on what position the thermostat was in at the time of failure, the engine may run either hot or cold. Both of these states would cause unusual temperature gauge readings. Additionally, engine performance will be a little off-peak. However, neither of these symptoms are very noticeable except in extreme conditions.

What Are the Causes of Code P0597?

In most cases, the cause behind the OBD-II scanner error code P0597 problem is a loose or corroded connector. The thermostat itself failing is another somewhat common cause. Less commonly, the engine management computer may fail and cause the code to set. These are some of the possible causes of the code:

  • A thermostat connector is corroded, loose or damaged.
  • The wiring connecting the thermostat to the computer has a short or open circuit.
  • There is a coolant leak.
  • The thermostat is faulty and needs repair or replacing.
  • The engine control computer is faulty.

How Do I Fix Trouble Code P0597?

Since the code is most likely to be caused by a faulty or corroded connector, that is the best starting point. Follow these steps to diagnose and fix P0597:

  • Inspect the electrical thermostat connector.
  • Remove any corrosion using baking soda and scraping.
  • Replace the connector using electro grease and confirm a tight connection.
  • Check the coolant level in the radiator. If it is low, the thermostat will overheat.
  • If the coolant level is low, check for any leaks.
  • Check the electrical resistance values of the thermostat. This procedure is model-specific. You will need the service manual for your vehicle.

These steps will fix this code in most situations. The service manual for your vehicle may have additional model-specific recommendations. Within-tolerance values are different for each vehicle, so make sure you have the appropriate information for your make and model.

With an infrared temperature probe and a volt/ohmmeter, you can check the thermostat and the engine control (Motronic) computer. These are the general steps, but they may vary by vehicle:

  • Check the engine temperature with the probe.
  • With the engine running, check the voltage of the harness on the engine control computer side. If there is no voltage or it is not within the tolerable range, the control computer needs to be replaced.
  • Do the same for the thermostat side of the harness. If there is no voltage or it is out of range, replace the thermostat.

This code is fairly easy to diagnose if you have the right tools. From there it is simply a case of replacing the faulty component.

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