Fault code P0599 – thermostat heater control circuit high

Fault code P0599 is called “Thermostat Heater Control Circuit High” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0599

Error code P0599 is a generic code that is defined as “high signal level in the thermostat heater control circuit”. It is set when the Powertrain Control Module (PCM) detects a high signal level in the motorized thermostat heater control circuit.

Fault code P0599 – thermostat heater control circuit high

An electronically controlled thermostat uses wax pellets and a spring to control its operation just like a conventional thermostat. But the wax pellets are supplemented by a heater element controlled by the PCM to control its operation depending on driving conditions.

The PCM uses data from engine speed, throttle position, intake air temperature and engine coolant. It determines operating conditions and the appropriate thermostat setting. Which will bring the engine temperature into a range where emissions can be reduced without sacrificing fuel economy or engine efficiency.

For an electronically controlled thermostat to work properly, the electrical circuits must be in perfect working order.

When the PCM detects a high signal level in the thermostat control circuits, it realizes that it cannot control effectively. This will set the code P0599 and the warning light on the instrument panel will illuminate.

Symptoms of vehicle malfunctions

The main signal that an error P0599 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. Check engine control lamp on the control panel will light up (the code will be recorded in the memory as a fault).
  2. High temperature reading if the thermostat has failed in the closed position.
  3. Low temperature reading if the thermostat has failed in the open position.

Error P0599 is considered serious because the engine can overheat, which can cause damage to its components. If this code is detected, it is recommended that the fault be repaired as soon as possible.

Factors that can cause this error code

The error code P0599 can mean that one or more of the following problems have occurred:

  • Faulty thermostat.
  • Loose or corroded connector.
  • Damaged, burned, or shorted wiring.
  • Coolant leakage.
  • Sometimes the cause is a faulty PCM module.

How to fix or reset OBD-2 code P0599

Some suggested steps for troubleshooting and fix the error code P0599:

  1. Read all stored data with an OBD-II scan tool.
  2. Check the coolant level in the radiator and expansion tank.
  3. Remove the electrical connector from the thermostat and check for corrosion and any damage.
  4. Measure the resistance at the connector.
  5. Inspect and test the wiring related to the thermostat.

Diagnose and repair of problems

The first step in the troubleshooting process for any malfunction is to review the Technical Service Bulletins (TSB). Check code P0599 for known problems with your particular vehicle.

Next, conduct a visual inspection of the wiring and connectors, and repair or replace any that are damaged, burned out, or damaged. Use a multimeter to check the voltage and ground circuits on the thermostat heater and PCM.

If no voltage is detected, check the system relays and associated fuses. Replace defective or blown fuses if necessary.

Test the thermostat heater control circuits and system components following the manufacturer’s instructions. If voltage and ground are not detected on the thermostat heater, check the corresponding circuit on the PCM connector.

If voltage is detected there, there is most likely an open circuit between the thermostat and the PCM. If no voltage is detected, the PCM may have failed or a programming error may have occurred.

It is a good idea to check the coolant level in the radiator. Low coolant level can cause the electronic thermostat to overheat and consequently set the error P0599.

On which vehicles does this problem occur most frequently

Fault code P0599 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Audi (A8)
  • BMW (E46, X3)
  • Chevrolet (Aveo, Cruze, Orlando, Sonic, Spark)
  • Citroen (C4)
  • Mercedes-Benz
  • Opel (Astra, Corsa, Mokka)
  • Peugeot (207, 308, 408)

Fault code P0599 can sometimes be found with other errors. The most common are the following: P0597, P0598.


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