P0599 OBD-II Trouble Code: Thermostat Heater Control Circuit High

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Cost of diagnosing the P0599 code

P0599 is a diagnostic trouble code (DTC) for "Thermostat Heater Control Circuit High". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $114.99 . Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.0 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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Toyota $114.99 $20.0
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P0599 code definition

The P0599 code is stored when the PCM detects an abnormally high voltage reading from the thermostat heater unit.

What the P0599 code means

The thermostat heater unit requires inputs from a variety of temperature sensors and engine sensors in order to adjust engine coolant flow and achieve optimal engine temperature. This entire system of sensors and electric components is called the thermostat heater control circuit. If the PCM registers a voltage reading that is too high compared to the manufacturer’s specifications for the thermostat heater control circuit, it will store the P0599 code.

What causes the P0599 code?

There are many things that can be behind the P0599 code. Those include:

  • Defective circuitry components, such as wiring, connectors, or harnesses that are corroded, loose, or damaged in some way
  • Defective thermostat
  • Defective control housing
  • Low engine coolant
  • Overheated engine
  • Prematurely operating cooling fan
  • Faulty PCM, in rare cases

What are the symptoms of the P0599 code?

The symptoms that can be present with the P0599 code can vary greatly depending on the position of the thermostat at the time of the malfunction. In addition to the illuminated Check Engine lamp, vehicle owners may observe an overheating engine in cool weather, or an engine that cannot reach optimal temperature very quickly in warm weather. If the thermostat malfunction happens after the engine is already at the correct temperature, there may not be any noticeable symptoms at all.

How does a mechanic diagnose the P0599 code?

The P0599 code should first be verified with an OBD-II scanner. After that, the mechanic should first check the coolant level, and be sure that there are no leaks that need repairing. After this step, and every sequential step, it is important to clear the P0599 code from the PCM, and retest the system. This process allows the mechanic to ensure that no unnecessary repairs are performed. The mechanic should then continue checking all components, including all electrical wiring, connectors, and harnesses, as well as the thermostat itself, until the faulty component is discovered.

Common mistakes when diagnosing the P0599 code

The most common mistake made with diagnosing the P0599 code is to replace the thermostat itself, rather than checking the circuitry for malfunctioning electrical parts. Often, leaking engine coolant may cause wiring and other components to become corroded. In this case, the source of the leak should also be repaired.

How serious is the P0599 code?

While the vehicle may show no symptoms at all when the P0599 code is stored, this code does represent a serious issue that could lead to major drivability issues later. A vehicle with this code stored in the PCM should be repaired as soon as possible.

What repairs can fix the P0599 code?

There are many ways that a technician can address a P0599 code detection. They include:

  • Replace defective circuitry components
  • Replace a defective thermostat
  • Replace or repair defective control housing
  • Refill low engine coolant
  • Replace a faulty PCM, in rare cases

Need help with a P0599 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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