Error Code P0599 is defined as Thermostat Heater Control Circuit High. This code is a generic trouble code that applies to all vehicles equipped with OBD-II system, especially those models made since 1996 up to present and with the same thermostatic control. It’s particularly common among German manufactured vehicles, such as Audi, BMW, Jaguar, Mercedes Benz, Mini, Opel, and Volkswagen.  Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

This fault refers to a problem in the system where the control voltage is nonexistent. An electronically controlled thermostat is composed of multiple temperature and load sensors, a computer program, and a control housing with a built-in thermostat.

The function of the thermostat is to control the flow of the coolant or temperature of the engine electronically, part throttle improves fuel economy and reduces emissions. On the other hand, reducing the temperature under load will increase power. It monitors and controls the temperature of the engine to ensure it doesn’t overheat or suffer possible problems.

If the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) determines a fault in the engine’s thermostat, specifically if it’s experiencing a problem with the control voltage, then it will store this code. For Error Code P0599, this means a fault has occurred within the system caused by high control voltage.

Other related error codes include:

Common Symptoms

Symptoms of this code usually vary depending on the position of the thermostat when the failure occurred. However, in most cases, there are usually no noticeable difference in the vehicle’s performance and drivability. In some cases, symptoms will be:

  • Temperature gauge reading is abnormally high
  • Overheating engine in cool weather
  • Engine unable to reach optimal operating temperature quickly in warm weather

In some cases where the thermostat malfunctions after the engine is already on the right operating temperature, drivers may not notice any signs or symptoms for this error code.

Possible Causes

There are many possible causes for this code, and in many cases, it’s caused by a problematic electrical connector. Thus, corroded or loose connectors and wires are the common causes for this code. Other possible causes include:

  • Defective circuitry components (corroded, damaged or loose connectors, harnesses, wirings)
  • Faulty control housing
  • Faulty thermostat
  • Overheated engine
  • Low engine coolant level
  • Operating the cooling fan prematurely
  • Defective PCM (rare)

How to Check

The first thing you should do in diagnosing this code is removing and inspecting the electrical connector. Corrosions can be removed using baking soda or by scraping. Then, apply electro grease and confirm the tight connection.

Next, check the coolant level in the radiator. Low coolant level can activate this code as it means can cause the electronic thermostat to overheat.

Next, remove the electrical connector and then check the resistance values on the thermostat. In this step, you will need a service manual, or you can search online for the specific diagnostic steps and repair procedures suggested by the vehicle’s manufacturer.

By checking through the service manual, you will be able to identify the pins and their location, colors of wires, pin values in volts as set temperature and resistance. These values, of course, vary from one manufacturer to another, and size of the engine.

In most cases, you will need an infrared probe and volt-ohmmeter to diagnose this code further.

Advanced Diagnosis

Next, using the temperature probe, check the temperature of the engine.

Start the engine and then test the voltage Motronic side of the harness (refer to the instructions from the service manual). If it’s within the limits, then continue testing. If there is no voltage or voltage is out of range, then the Motronic Unit must be replaced.

Next, compare the resistance on the thermostat side of the harness at the thermostat. If resistance is out of range, the thermostatic unit must be replaced.

How to Fix

Depending on the diagnosis, common repairs for this code include:

  • Repair or replacement of defective circuitry components
  • Replacement of thermostat
  • Repair or replacement of faulty control housing
  • Replacement of PCM (rare)

Diagnosing this code can be simple if you have the tools. If you don’t have the right tools, then it’s best to take your vehicle to a good shop for a thorough diagnosis.

The most common misdiagnosis for this code is replacing the thermostat, instead of checking the circuitry for defective and malfunctioning electrical parts.

Also, a leaking engine can cause corrosion to the connectors and wirings. In which case, the source of the leak must be located and repaired, if not replaced.

While this code is unlikely to cause serious damage to the engine, overheating can result to problems to various components. Thus, it’s best to have this code addressed by a certified technician as soon as possible.