P0599 – Thermostat heater control system -circuit high

By Stephen Darby (Contact Me)
Last Updated 2019-11-03
ASE Master Tech

Table of Contents

1. What Does Code P0599 Mean?
2. What are the common causes of code P0599 ?
3. What are the symptoms of code P0599 ?
4. How do you troubleshoot code P0599 ?
5. Codes Related to P0599
6. Get Help with P0599

What Does Code P0599 Mean?

When a malfunction indicator lamp is accompanied by a trouble code P0599, it means that the powertrain control module (PCM) has detected a malfunction (high or excessive voltage) in the thermostat heater control circuit of the engine cooling system. Only vehicles equipped with an electronically heated thermostat should exhibit a code P0599. In this case, the malfunction may be electrical or mechanical.

Typically, engine coolant temperature is monitored (by the PCM) with one or more engine coolant temperature sensors (ECT). The ECT sensor/s consist of a brass (or plastic) housing which has a thermal resistor inside. The ECT sensor housing is designed so that it may be threaded into a coolant passage of the engine block, cylinder head, or intake manifold. When the thermostat opens, coolant flows across the tip of the ECT sensor (where the thermal resistor is). The ECT sensor is provided a reference voltage (usually 5-volts) and a ground. The sensor completes the circuit. As engine coolant temperature increases, the resistance of the thermal resistor decreases. The result of this decrease in circuit resistance is higher voltage being input to the PCM. When engine coolant temperature decreases, a reverse action occurs and circuit voltage decreases. The PCM interprets these variations in circuit voltage as changes in engine coolant temperature.

The spring-loaded thermostat operates by opening when engine coolant reaches a certain temperature (usually 180-degrees F). A sealed capillary is filled with wax and a rod is inserted into the wax. The rod is then attached to a plunger in the thermostat. When the wax becomes hot, it expands inside the tightly sealed capillary; pushing the rod upward and opening the plunger. With the plunger open, coolant may flow freely between the engine and radiator.

The electronically heated thermostat uses an integrated heater element to increase capillary pressure in a more controlled manner. The heater element increases capillary temperature much faster than engine coolant. This allows the PCM to control thermostat operation regardless of coolant temperature or coolant level.
Every automobile utilizes some type of thermostat to regulate engine coolant flow and maximize performance. The electronically controlled thermostat simply gives the PCM an added level of control over the flow of engine coolant in certain situations.

Thermostat heater control voltage is constantly monitored by the PCM during engine operation. If the PCM detects a degree of thermostat heater control voltage that exceeds the maximum threshold, a code P0599 will be stored and a malfunction indicator lamp (MIL) may be illuminated. Multiple ignition cycles (with a failure) may be required for MIL illumination.

What are the common causes of code P0599 ?

Thermostat heater element burned out (very common)

Defective primary or secondary ECT sensor

Faulty ambient temperature sensor

Shorted or open circuits or connectors

Bad thermostat

Low engine coolant

What are the symptoms of code P0599 ?

Rich exhaust

Cold start drivability issues

No heat inside the passenger compartment

Diminished engine performance

How do you troubleshoot code P0599 ?

In order to diagnose a code P0599, a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of reliable vehicle information will be required.

Searching technical service bulletins (TSB) that resemble the code stored, vehicle (year, make, model, and engine), and symptoms exhibited could net a faster diagnosis. TSB information can be found in your vehicle information source.

Prior to clearing stored codes, use the scanner (connected to the vehicle diagnostic port) to retrieve all stored codes and pertinent freeze frame data. Write the information down (in case the code/s proves to be an intermittent one). Once codes are recorded, clear the codes and test drive the vehicle. One of two things will likely occur; the code will be restored or the PCM will enter readiness mode. If the code is intermittent the PCM may enter readiness mode. If this occurs, the condition which caused the P0599 to be stored may need to worsen before an accurate diagnosis can be made. Continue with the diagnosis if the code is reset.

Use your vehicle information source to obtain connector face views, connector pinout charts, component locator charts, wiring diagrams, and diagnostic flow charts (pertaining to the code and vehicle in question). Perform a visual inspection of related wiring and connectors and repair or replace wiring that has been cut, burned, or damaged.

The DVOM may be used to test voltage and ground circuits at the thermostat heater or PCM. If no voltage is detected, check system relays and related fuses. Replace defective relays and/or blown (or otherwise defective) fuses as required. Test thermostat heater control circuits and system components following manufacturer’s guidelines. If voltage and ground are not detected at the thermostat heater, test the corresponding circuit at the PCM connector. If voltage is detected there, suspect an open circuit between the component in question and the PCM. If voltage is not discovered there, suspect a defective PCM or a programming error.

Codes Related to P0599

P0597 Thermostat Heater Control Circuit/Open

P0598 Thermostat Heater Control Circuit Low

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