The P0599 trouble code, officially known as “Thermostat Heater Control Circuit High,” is a diagnostic code that signifies an issue in the thermostat’s electronic control system. This code is part of the OBD-II system, which monitors vehicle performance and emissions. Specifically, P0599 indicates that the thermostat heater circuit is experiencing a high voltage condition. This malfunction can disrupt the engine’s ability to regulate temperature effectively, potentially leading to overheating or inefficient performance.
| P0599 Code Meaning | P0599 Code Common Causes |
|---|---|
| Thermostat heater circuit voltage too high | Faulty thermostat |
| Malfunction in the electronic thermostat control | Wiring harness issues (open/short circuits) |
| Failure in the powertrain control module (PCM) | Corroded or loose connectors |
| Coolant flow irregularities | Low coolant levels or leaks |
Understanding P0599: Symptoms and Solutions
Symptoms of P0599
- Check Engine Light (CEL) illuminated
- Engine overheating or running cooler than usual
- Reduced fuel efficiency
- Erratic engine temperature readings
- Cooling fan running excessively
Technical Explanation
Modern vehicles use electronically controlled thermostats to regulate engine temperature for optimal performance and emissions control. The thermostat heater circuit within this system adjusts the thermostat’s operation based on signals from the PCM. When the PCM detects abnormally high voltage in this circuit, it triggers the P0599 code. This could result from a faulty thermostat, damaged wiring, or issues with the PCM itself.
Diagnosing and Fixing P0599
Step 1: Inspect Electrical Connections
Symptoms: Loose or corroded connectors can cause intermittent electrical faults.
Diagnosis:
- Locate the thermostat and its electrical connector.
- Inspect for corrosion, damage, or loose connections.
- Clean corroded terminals with an appropriate cleaner and apply dielectric grease.
Solution: Secure all connections and replace damaged connectors if necessary.
Cost Estimate: $10–$50 for cleaning materials or replacement connectors.
Warnings: Always disconnect the battery before working on electrical components.
Step 2: Test Thermostat Heater Resistance
Symptoms: Faulty thermostats may fail to regulate engine temperature properly.
Diagnosis:
- Use a multimeter to measure resistance across the thermostat heater terminals.
- Compare readings with manufacturer specifications (found in service manuals).
Solution: Replace the thermostat if resistance is out of range.
Cost Estimate: $100–$300 for a new thermostat (varies by vehicle make/model).
Warnings: Ensure proper coolant handling when replacing thermostats to avoid spills or burns.
Step 3: Check Wiring Harness
Symptoms: Open or short circuits in wiring can disrupt signal transmission.
Diagnosis:
- Inspect wiring between the PCM and thermostat for visible damage.
- Use a voltmeter to check for continuity and proper voltage levels.
Solution: Repair or replace damaged wiring as needed.
Cost Estimate: $50–$200 depending on labor and parts.
Warnings: Avoid splicing wires without proper insulation techniques to prevent future issues.
Step 4: Verify PCM Functionality
Symptoms: Persistent P0599 code despite fixing other issues may indicate PCM failure.
Diagnosis:
- Use an advanced OBD-II scanner to check PCM signals related to the thermostat heater circuit.
- Test PCM outputs using manufacturer-specific procedures.
Solution: Replace or reprogram the PCM if diagnosed as faulty.
Cost Estimate: $500–$1,500 depending on vehicle make/model and whether reprogramming is required.
Warnings: PCM replacement often requires professional assistance due to programming needs.
Step 5: Address Coolant System Issues
Symptoms: Low coolant levels can exacerbate thermostat problems.
Diagnosis:
- Check coolant levels in the radiator and reservoir.
- Inspect for leaks around hoses, radiator, and water pump.
Solution: Refill coolant or repair leaks as necessary.
Cost Estimate: $20–$150 depending on coolant type and repair complexity.
Warnings: Use only manufacturer-recommended coolant types to avoid compatibility issues.
Frequently Asked Questions About P0599
- What does error code P0599 mean?
The P0599 code indicates a high voltage issue in the thermostat heater control circuit. - Can I drive with a P0599 code?
No, driving with this code can lead to engine overheating or other performance issues. - How do I reset a P0599 code?
Fix the underlying issue first, then use an OBD-II scanner to clear the code. - What tools are needed to diagnose P0599?
You’ll need a multimeter, OBD-II scanner, and possibly an infrared thermometer. - Is replacing the thermostat expensive?
Costs range from $100–$300 depending on your vehicle. - Can low coolant cause P0599?
Yes, insufficient coolant can lead to overheating and trigger this code. - Do all vehicles have electronic thermostats?
No, only certain modern vehicles use electronically controlled thermostats. - When should I consult a professional?
If you lack diagnostic tools or suspect PCM failure, seek professional help.
Conclusion
The P0599 trouble code is a critical indicator of issues within your vehicle’s thermostat heater control circuit. While diagnosing and repairing this problem may seem daunting, following systematic steps can help resolve it effectively. Always prioritize safety during repairs and consult professional mechanics when necessary. Addressing this issue promptly ensures your engine operates efficiently while avoiding costly damage down the line.