P0599 — Thermostat Heater Control Circuit High #

Note: The code description may vary depending on the vehicle manufacturer. Always refer to your OEM (Original Equipment Manufacturer) documentation for the most accurate wording.

• Thermostat Heater Control Circuit High (Generic / GM / Ford / Chrysler)
• Engine Coolant Thermostat Heater Control Circuit High (Volkswagen / Audi)
• Thermostat Heater Control Circuit High Input (Hyundai / Kia)
• Heater Control Circuit High (BMW / Mini)

Summary of Diagnostic Trouble Code P0599 #

P0599 is a generic powertrain code indicating that the engine control module (ECM) or powertrain control module (PCM) has detected a higher-than-expected voltage or resistance in the thermostat heater control circuit. This circuit is responsible for managing the electronic thermostat’s heating element, which helps regulate engine temperature precisely. Modern engines often use electronically controlled thermostats for more efficient warm-up and emissions control. When this circuit goes “high,” it typically means there is an electrical fault, such as an open circuit, high resistance, or failed component. The vehicle’s computer monitors this system closely to ensure optimal operation and emissions compliance.

Common Symptoms for Code P0599 #

While P0599 might not always trigger dramatic symptoms, you may notice subtle changes in your vehicle’s behavior. The most frequent signs include:

• Illuminated Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL)
• Engine takes longer to reach normal operating temperature
• Poor heater performance (especially in cold weather)
• Reduced fuel efficiency
• Possible engine temperature fluctuations or erratic gauge readings
• Emission test failure due to improper warm-up

Probable Causes for Code P0599 #

Diagnosing P0599 begins with understanding the most likely root causes. In order of frequency, they include:

1. Faulty thermostat heater element (internal open circuit or high resistance)
2. Damaged or corroded thermostat wiring/connectors (especially near the thermostat housing)
3. Blown fuse or relay associated with the thermostat control circuit
4. PCM/ECM software or internal fault (rare, but possible if all else checks out)
5. High resistance in the ground circuit (poor connection or corrosion)

Step-by-Step Diagnostic Process for Code P0599 #

Tackling P0599 effectively requires a systematic approach. Here’s a step-by-step diagnostic roadmap:

• Verify the presence of P0599 and check for related codes using a scan tool.
• Visually inspect the thermostat housing, wiring harness, and connectors for damage, corrosion, or loose connections.
• Test the thermostat heater circuit with a digital multimeter for proper voltage and resistance values (refer to OEM specs).
• Check for blown fuses or faulty relays controlling the thermostat heater circuit.
• Backprobe the connector at the thermostat while commanding the heater ON with a scan tool to verify ECM/PCM output.
• Test for continuity and excessive resistance in the ground side of the circuit.
• If all physical checks are good, consider a possible PCM/ECM issue and consult OEM service bulletins.

Possible Repairs for Code P0599 #

Once the root cause is found, addressing P0599 usually involves one or more of the following repairs:

• Replace a faulty or failed thermostat assembly (with integrated heater)
• Repair or replace damaged or corroded wiring/connectors
• Replace blown fuses or faulty relays related to the circuit
• Clean and secure ground connections for the thermostat circuit
• Update or reprogram the PCM/ECM if a software issue is found (rare)

Is It Safe to Drive with Code P0599? #

Yes, but with limitations. While P0599 usually won’t cause immediate drivability issues, running an engine that takes too long to warm up—or never reaches optimal temperature—can increase fuel consumption and emissions over time. In cold weather, poor heater performance can also be uncomfortable. Prolonged operation without addressing the issue may lead to engine wear or emission system problems. It’s best to fix the cause of P0599 at your earliest convenience.

Vehicles Most Commonly Affected by Code P0599 #

P0599 is a generic OBD-II code, but it’s most frequently reported on vehicles that use electronically controlled thermostats. Models include:

• Chevrolet Cruze, Malibu, and Sonic
• Ford Focus and Fiesta (recent models)
• Hyundai Elantra, Sonata, and Kia Optima
• Volkswagen Jetta, Passat, and Golf
• BMW 3 Series and Mini Cooper (with electronic thermostats)
• Chrysler 200, Dodge Dart, and Jeep Compass

Common Mistakes & Best Practices for Code P0599 #

To avoid frustration (and unnecessary costs), keep these tips in mind:

• Don’t replace the thermostat without verifying wiring and connectors; many issues are electrical, not mechanical.
• Don’t ignore corrosion—even minor oxidation at connectors can cause high resistance.
• Always use OEM or high-quality replacement parts to ensure proper function.
• Clear the code and retest after each repair to confirm the fix.
• Consult OEM wiring diagrams for accurate circuit identification.

Express FAQ for Code P0599 #

Still have questions? Here are quick answers to the most common concerns:

• Q: Can a stuck-open thermostat trigger P0599?
  A: Not directly. P0599 is triggered by an electrical fault in the heater circuit—not by the physical position of the thermostat. However, a failed thermostat may have an internal electrical issue that also affects its operation.
• Q: Will replacing just the thermostat housing fix P0599?
  A: Only if the internal heater is faulty. Always test the circuit first; if the wiring or PCM is at fault, replacing the housing alone won’t resolve the issue.