P0599 Code: Here's How to Solve It Fast

The P0599 trouble code is a generic powertrain code related to the engine's thermostat heater control circuit. When your vehicle's onboard diagnostic system detects a voltage reading that is higher than expected in this circuit, it triggers the P0599 code. This issue primarily affects the engine's cooling system and can lead to various performance problems if left unaddressed.

P0599 Code Meaning	P0599 Code Common Causes
Thermostat Heater Control Circuit High	Faulty thermostat heater
Voltage reading above specified range	Damaged wiring or connectors
Issue with electronically controlled thermostat	Corroded electrical connections
Problem in engine cooling system control	Malfunctioning engine control module (ECM)
Potential coolant temperature regulation failure	Low coolant level or coolant leaks

Understanding the Thermostat Heater Control Circuit

The thermostat heater control circuit is an integral part of modern engine management systems, particularly in vehicles manufactured by German automakers like BMW, Mercedes-Benz, Audi, and Volkswagen. This sophisticated system allows for precise control of engine temperature, which is crucial for optimal performance, fuel efficiency, and emissions control.

Symptoms of P0599 Code

When the P0599 code is present, you may experience the following symptoms:

1. Illuminated check engine light
2. Engine overheating in cool weather
3. Slow engine warm-up in warm weather
4. Erratic temperature gauge readings
5. Decreased fuel efficiency
6. Poor engine performance
7. Cooling fans running constantly

It's important to note that in some cases, there may be no noticeable symptoms other than the check engine light. However, this doesn't mean the issue should be ignored, as it can lead to more severe problems over time.

Technical Explanation of P0599

The P0599 code specifically indicates that the voltage in the thermostat heater control circuit is higher than the manufacturer's specified range. This circuit is responsible for controlling an electrically heated thermostat, which allows for more precise regulation of engine coolant flow compared to traditional mechanical thermostats.

The system works by using various temperature and load sensors to determine the optimal engine temperature. The engine control module (ECM) then adjusts the thermostat's operation through the heater control circuit. When the ECM detects a voltage that's too high in this circuit, it triggers the P0599 code.

Diagnosing the P0599 Code

Proper diagnosis of the P0599 code requires a systematic approach and some specialized tools. Here's a step-by-step guide for diagnosing this issue:

1. Verify the code: Use an OBD-II scanner to confirm the P0599 code and check for any additional related codes.
2. Check the coolant level: Ensure the coolant is at the proper level and there are no visible leaks.
3. Inspect wiring and connectors: Look for any signs of damage, corrosion, or loose connections in the thermostat heater control circuit.
4. Test the thermostat: Use a multimeter to check the resistance of the thermostat heater. Compare the readings to the manufacturer's specifications.
5. Check voltage readings: With the engine running, use a voltmeter to measure the voltage at the thermostat heater connector. Compare this to the specified range in the vehicle's service manual.
6. Scan for live data: Use an advanced scanner to monitor the thermostat heater control circuit's operation in real-time.
7. Perform a voltage drop test: Check for excessive voltage drop across the circuit, which could indicate resistance issues.
8. Test the ECM: If all other components check out, the issue may lie with the ECM itself.

Solutions for P0599 Code

Once you've identified the root cause of the P0599 code, you can proceed with the appropriate solution:

Replacing the Thermostat

If the thermostat is faulty, it will need to be replaced. This process typically involves:

1. Draining the coolant
2. Removing the old thermostat
3. Installing the new thermostat with a new gasket
4. Refilling the cooling system with the correct type and amount of coolant
5. Bleeding the cooling system to remove any air pockets

Warning: Always allow the engine to cool completely before working on the cooling system to avoid burns from hot coolant.

Repairing Wiring Issues

If the problem lies in the wiring or connectors:

1. Clean any corroded connections using electrical contact cleaner
2. Repair or replace damaged wires
3. Ensure all connections are tight and secure
4. Apply dielectric grease to connections to prevent future corrosion

Addressing Coolant Leaks

If low coolant level is contributing to the issue:

1. Locate and repair any coolant leaks
2. Refill the cooling system with the correct coolant
3. Pressure test the system to ensure there are no remaining leaks

ECM Replacement or Reprogramming

In rare cases where the ECM is at fault:

1. Have the ECM tested by a professional to confirm the issue
2. Attempt to reprogram the ECM if possible
3. Replace the ECM if reprogramming doesn't resolve the issue

Note: ECM replacement or reprogramming should only be performed by qualified professionals with the proper diagnostic equipment.

Cost Estimates for P0599 Repairs

The cost to repair issues related to the P0599 code can vary widely depending on the root cause and the vehicle make and model. Here are some general estimates:

• Thermostat replacement: $150 - $400
• Wiring repairs: $100 - $300
• Coolant system repairs: $200 - $800
• ECM reprogramming: $150 - $300
• ECM replacement: $500 - $2,000+

These estimates include parts and labor but can vary based on your location and the specific vehicle.

Preventing P0599 Code Issues

To help prevent P0599 code issues in the future:

• Maintain proper coolant levels and perform regular coolant flushes
• Address any coolant leaks promptly
• Keep electrical connections clean and protected from corrosion
• Follow manufacturer-recommended maintenance schedules
• Use high-quality replacement parts when repairs are needed

When to Seek Professional Help

While some aspects of diagnosing and repairing P0599 code issues can be done by experienced DIY mechanics, there are situations where professional help is recommended:

• If you lack the necessary diagnostic tools
• When dealing with complex electrical issues
• If the problem persists after attempting repairs
• When working with high-voltage hybrid or electric vehicle systems
• If you're unsure about any aspect of the diagnosis or repair process

Remember, improper repairs can lead to more severe and costly issues down the line. When in doubt, consult a certified automotive technician.

Conclusion

The P0599 code, while not immediately critical, should not be ignored. It indicates a problem with your vehicle's engine cooling system control, which can lead to performance issues and potential engine damage if left unaddressed. By understanding the causes, symptoms, and solutions for this code, you can take appropriate action to keep your vehicle running smoothly and efficiently.

Remember to always prioritize safety when working on your vehicle, and don't hesitate to seek professional help when needed. With proper diagnosis and repair, you can resolve the P0599 code and ensure your engine maintains optimal operating temperature for peak performance and longevity.

Frequently Asked Questions About P0599

• Can I still drive my car with the P0599 code?
  While you may be able to drive short distances, it's not recommended. The code indicates a problem with engine temperature control, which could lead to overheating and engine damage.
• How urgent is it to fix the P0599 code?
  It's important to address this issue promptly. While not immediately critical, ignoring it can lead to more serious engine problems over time.
• Will the P0599 code clear itself?
  The code may clear temporarily if the issue resolves, but it will likely return if the underlying problem isn't fixed. Proper diagnosis and repair are necessary for a permanent solution.
• Can a bad battery cause the P0599 code?
  While unlikely, a weak battery could potentially cause erratic voltage readings. However, this would typically trigger other codes as well.
• How long does it take to fix a P0599 code?
  Repair time can vary from 1-4 hours depending on the cause. Simple wiring repairs may be quick, while thermostat or ECM replacements can take longer.
• Is the P0599 code covered under warranty?
  It may be covered under powertrain warranty, depending on your vehicle's age, mileage, and the specific cause of the issue. Check your warranty terms or consult with your dealer.
• Can I replace the thermostat myself to fix the P0599 code?
  While possible for experienced DIYers, thermostat replacement can be challenging on some vehicles. Consider your skill level and the tools required before attempting this repair.
• Will a P0599 code cause my vehicle to fail an emissions test?
  In many regions, any active check engine light, including for P0599, will cause an emissions test failure. Resolving the issue is necessary to pass the test.