Fix P0599 Obd2 Code: Thermostat Heater Control Circuit High

Affiliate Disclimer: ObdCore is reader supported, if you click on a link and make a purchase, We may receive commissions from purchases made via our links at no additional costs to you.

How To Fix P0599 Obd2 Code?
Table of Contents

The Thermostat Heater Control Circuit High error code P0599 is described as Thermostat Heater Control Circuit High. The thermostat’s primary duty is to regulate the coolant flow or the engine’s temperature electronically. Using a part throttle helps to increase fuel efficiency while also lowering pollution.

P0599 Meaning:

The thermostat heater unit needs data from several temperature sensors and engine sensors to regulate engine coolant flow and achieve optimum engine temperature.

P0599 Meaning:

The thermostat heater control circuit refers to the complete set of sensors and electric components. PCM will record the P0599 error code if the voltage reading for the thermostat heating control circuit is greater than the manufacturer’s standards.

OBD-II Code P0599: Symptoms, Causes & Diagnose


The P0599 code may cause a wide range of symptoms based on the thermostat’s location at the time of the failure. Overheating engines in cold weather or a slow-to-warm engine in hot weather might also indicate a problem with the vehicle’s cooling system. Symptoms may not be visible if the thermostat breakdown occurs while the engine temperature is correct.


A variety of factors may cause the P0599 error code. These are some examples:

Corroded, loose, or otherwise damaged wire, connections, or harnesses in the circuit.

  • Thermostat malfunction.
  • The control housing has a problem.
  • A lack of engine coolant.
  • Excessive heat from the engine.
  • Operating a cooling fan too early.
  • Rarely a faulty PCM will be the cause of the issue.

How to Diagnose?

You should use the OBD-II scanner to verify the P0599 code. A cooling system inspection follows this to ensure no leaks exist and may be repaired. You must clear the P0599 code from the PCM, and the system retests after each subsequent step. 

So that the technician doesn’t waste time doing fixes that don’t need to be done. At this point, the technician should inspect every component, including the thermostat and the wire harnesses that link to it, to identify the problematic one.

Tools You Need:

  • FIXD
  • Multimeter
  • What Everybody Should Have in Their Tool Bag


Remove the electrical connection and do a visual inspection of it. Scrape or use baking soda to get rid of any rust build-up. Tighten the connection with an o-ring and electrical grease. Inspect the radiator’s coolant level. The electronic thermostat will overheat if there is not enough coolant to clear the code. 

Check the thermostat’s resistance readings after removing the electrical connection. A service manual or internet research is required for this process. As a result, this data must contain information on PINs (and where they are located), wire colors, voltage values at various temperatures, and resistances. 

Depending on the size of the engine, these numbers and the diagnostic flow may change from one manufacturer to the next. The volt/ohmmeter and infrared temperature probe are also required. 

The engine’s temperature may be measured using a thermometer. Test voltage on the harness’s Motronic side using the instructions provided.

If it’s within acceptable parameters, continue testing. Replace the Motronic device if the voltage was not present or out of range. The thermostat side of the harness should be compared to the thermostat side of the saddle. 

The thermostatic unit should be replaced if the resistance is outside the acceptable range. Taking the car to a reputable repair shop will help if you don’t have the essential equipment on hand. With the right equipment, it’s a straightforward procedure.

A common mistake you must avoid when diagnosing P0599:

One of the most typical diagnostic blunders is replacing the thermostat without inspecting the circuits for broken electrical components. When engine coolant leaks, it may ruin wiring and other vehicle parts. In this instance, it’s essential to fix the leak’s source.

How to fix the P0599 Obd2 Code?

How to fix P0599?

In some instances, this code may need the following fixes:

  • Repair or replacement of circuitry components that have become faulty.
  • Thermostat replacement.
  • A malfunctioning control housing has to be repaired or replaced.
  • As an alternative (rare).

If you have the right tools, this code can be easily diagnosed. If you don’t have the necessary equipment, the best course of action is to get your car diagnosed by a qualified mechanic.

Misdiagnosis of this code involves changing the thermostat rather than examining for damaged or malfunctioning electrical components in the circuitry.

Connectors and wires might corrode as a result of an engine leak. If this is the case, the leak’s origin must be identified and fixed or replaced entirely.

Tips for further avoiding P0599 obd2 error:

Regardless of whether or not the P0562 code is present, the check engine light will flash regardless of what the driver does. Pay close attention to how much petrol you utilize daily. 

If you’re using more gas than usual, you should take your vehicle to the technician. Following the manufacturer’s recommendations for gasoline octane and having your vehicle’s oil and other fluids checked regularly are two essential safety precautions to take.


Cost of diagnosing the P0599 code:

The “Thermostat Heater Control Circuit High” DTC code P0599 indicates a malfunctioning thermostat. An experienced technician will be necessary to determine the exact cause of this code being activated in your case. 

Check Engine Light diagnostics may be performed at your home or business for $114.99 by one of our licensed mobile experts. You will get an upfront quotation for the suggested remedy and a $20.0 credit toward the repair after determining the issue. Our 12-month/12,000-mile guarantee covers all of our repairs.

How serious is the P0599 code?

The P0599 code indicates a severe malfunction that might result in significant drivability issues in the future, even if the vehicle does not exhibit any symptoms. In contrast, the code is being recorded on the computer. If this code is present in a car, the PCM is recommended to be repaired as soon as possible.

If you see a P0599 code on your OBDII scanner, schedule an appointment with a technician as soon as possible.

Use a competent specialist to check the problem and complete all essential repairs.