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P0599 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


P0599 Thermostat Heater Control Circuit High


Like the thermostat in your home tells you how hot or cold it is inside, the thermostat in your vehicle tells your vehicle ECM (Electronic Control Module) the current temperature of your engine. 

There are many temperature sensors and engine sensors that collect information and send it to your car’s “brain” in the ECM.

Your ECM uses that input to adjust engine coolant flow and keep your engine at optimal operating temperature. The name for this entire system of sensors and electronic components is called the thermostat heater control circuit (THCC). 

Due to the sensitive nature of your car’s THCC, the PCM (Power Control Module) sometimes registers a high voltage reading compared to the manufacturer’s specifications. When that happens, your vehicle will code a P0599 and your Check Engine Light will illuminate.


There are many potential causes for a THCC High reading on a diagnostic tool. Here’s a summary of the most common ones:

  • Defective components of your vehicle’s wiring, connectors, or harnesses that are loose, corroded or physically damaged (i.e. chewed/worn/cut)
  • Defective thermostat unit/module (should check all other potential causes before replacing)
  • Defective control housing (cracked housing can cause malfunctions)
  • Low engine coolant (one of the first things you or a technician should verify)
  • Overheated engine (when your engine’s heat exceeds the sensors capabilities it will often trigger a high voltage trouble code)
  • Prematurely operating cooling (coolant flow adjustment is off balance and feeding too much or too little to the engine when the temperature is already within optimal range)

Also Read: Car Overheating When Idle


Verifying the symptoms of a P0599 code can be tricky. Depending on the position of the thermostat at the time of malfunction, you may not even notice any problems with your engine until it starts overheating.

You are more likely to see an illuminated Check Engine lamp first, or you may get an engine overheating temperature warning in cool weather.

Additionally, you may observe your engine cannot reach optimal temperature quickly in warm weather.

Worst of all, if the thermostat malfunction happens after the engine is already at the correct temperature, there may not be any noticeable symptoms at all before your engine begins overheating.


Here’s the order of process for diagnosing the cause of a P0599 code.

  1. First, the trouble code should first be verified with an OBD-II scanner.
  2. Next, the mechanic should check the coolant level to make sure that there is sufficient engine coolant present.
  3.  Once the coolant level is verified,  your technician will verify there are no leaks in the cooling system that need repairing.
  4. After checking the  coolant level and coolant flow system, and every sequential step, the technician will clear the P0599 code from the PCM and retest the system.
  5. Finally, the technician will check the connectors and wiring of the THCC for any signs of wear or damage.

Taking things step by step and checking all probable causes is the best way to ensure customers don’t pay for expensive repairs that don’t actually resolve their THCC problem.

Common mistakes

Many technicians just pull and swap the thermostat without verifying if there is another system issue in the wiring or cooling system.

A smart technician knows to check for leaking engine coolant before replacing any components, as the composition of engine coolant may cause wiring and other components to become corroded.

Once they have determined the actual cause of the error code, it’s easier to make effective repairs so the customer’s vehicle is fixed right the first time.

How serious

The problem with a P0599 scenario is your vehicle may not show any symptoms or problematic operation even when the code is stored.

Seeing a P0599 trouble code on your OBDII scanner.  Here’s a great one we use daily, It should be a red alert that sends you into the nearest repair shop to get the problem resolved as soon as possible. A P0599 code could lead to major drivability issues later, or cause expensive damage that requires extensive down time to repair.

Bottom line, if you see this code come up, you need to get your car in a shop right away. Waiting could have catastrophic results.

What repairs can fix the code

Depending on the source of the problem, fixing the issue is relatively simple:

  • Repair or replace defective circuitry components
  • Repair or replace a defective thermostat
  • Repair or replace defective control housing
  • Refill low engine coolant
  • Replace a faulty PCM (extremely rare problem to have)


If you see a P0599 code come up on your OBDII scanner, get your car in to be checked out right away.

A trained professional needs to see what’s causing the problem and ensure that all needed repairs are made to prevent further trouble.

P0599 Code – What Does It Mean & How To Fix It