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Engine-Codes – GMC – P0599 GMC

P0599 GMC Code - Thermostat Heater Control Circuit High

Quick Answer

The P0599 GMC code refers to an issue with the thermostat heater control circuit being detected as high by the vehicle's engine control module (ECM). Common causes include Faulty thermostat heater, Wiring or connector issues in the thermostat heater control circuit, Defective engine control module (ECM). Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means
P0599 GMC indicates Thermostat Heater Control Circuit High.
Common causes
Common causes include Faulty thermostat heater, Wiring or connector issues in the thermostat heater control circuit, Defective engine control module (ECM).
Severity
High - diagnose this code as soon as possible.
Can I drive?
Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
What is this?
Table of contents Created and updated by William Orellana on Sep 06, 2024
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Code P0599 GMC Description

The P0599 GMC code refers to an issue with the thermostat heater control circuit being detected as high by the vehicle's engine control module (ECM). The thermostat heater control circuit is responsible for regulating the temperature of the engine coolant by controlling the operation of the thermostat. When the ECM detects that the voltage in the thermostat heater control circuit is higher than normal, it triggers this diagnostic trouble code (DTC) to indicate a problem with the thermostat heater control system.

OBDII Code P0599 GMC - Thermostat Heater Control Circuit High
P0599 GMC Code - Thermostat Heater Control Circuit High

Common Causes of P0599 GMC

  1. Faulty thermostat heater
  2. Wiring or connector issues in the thermostat heater control circuit
  3. Defective engine control module (ECM)
  4. Low coolant levels
  5. Corrosion or damage to the thermostat heater control circuit components

Symptoms of P0599 GMC

  1. Engine overheating
  2. Inconsistent engine temperature readings on the dashboard
  3. Poor fuel efficiency
  4. Check Engine Light illuminated
  5. Engine running rough or stalling

How to Fix P0599 GMC

  1. Begin by diagnosing the issue using a scan tool to retrieve the fault code and identify the specific cause of the problem.
  2. Inspect the thermostat heater and its wiring for any signs of damage or corrosion. Replace any faulty components as needed.
  3. Test the thermostat heater control circuit for proper voltage and continuity. Repair or replace any damaged wiring or connectors.
  4. Clear the fault code from the ECM's memory using the scan tool and perform a test drive to verify that the issue has been resolved.
  5. If the problem persists, further diagnosis may be necessary to identify any underlying issues with the ECM or other related components.

Cost to Fix P0599 GMC

The cost of repairing a P0599 GMC code can vary depending on the specific cause of the issue and the labor rates at the auto repair shop. In general, the parts required for this repair, such as a new thermostat heater or wiring components, may range from $50 to $200. Labor costs can add an additional $100 to $300, depending on the shop's hourly rates and the complexity of the repair. Overall, the total repair cost for addressing a P0599 code may range from $150 to $500. It is advisable to consult with a professional mechanic for a more accurate estimate based on your vehicle's make and model.

What does this mean?

Tech Notes for P0599 GMC

The engine coolant thermostat heater has a mechanical fail-safe in case of an electrical condition with the engine coolant thermostat heater. The mechanical thermostat will open at approximately 104°C (220°F). The mechanical thermostat will cycle from approximately 104°C (220°F) to approximately 98°C (208°F). The heater is part of the thermostat. The engine coolant thermostat heater and the thermostat housing are serviced as an assembly.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0599 GMC Code

1. What does the OBDII code P0599 mean on a GMC?

P0599 indicates a "Thermostat Heater Control Circuit High," meaning the Engine Control Module (ECM) has detected a higher-than-expected voltage in the thermostat heater control circuit.

2. What are the common symptoms of code P0599 in a GMC vehicle?

Symptoms may include an illuminated check engine light, engine overheating, poor fuel economy, irregular engine temperature readings, or the heater not functioning properly.

3. What causes the P0599 code to appear on a GMC?

Common causes include a faulty thermostat heater, damaged wiring or connectors in the thermostat control circuit, a malfunctioning ECM, or a short circuit in the electrical system.

4. Can I drive my GMC with a P0599 code?

Driving with a P0599 code is not recommended, as it may lead to engine overheating or inefficient performance, potentially causing further damage to the engine.

5. How do I diagnose the P0599 code on my GMC vehicle?

Diagnosis involves checking the thermostat heater's resistance, inspecting the wiring and connectors for damage, testing the voltage in the thermostat control circuit, and verifying the ECM's functionality.

6. How can I fix the P0599 code on a GMC?

Repairs may include replacing a faulty thermostat heater, repairing or replacing damaged wiring or connectors, or reprogramming or replacing the ECM if necessary.

7. What tools are needed to troubleshoot the P0599 code?

You’ll need an OBDII scanner, a multimeter to test electrical connections, and potentially a repair manual specific to your GMC model.

8. How much does it cost to repair the P0599 code on a GMC?

Repair costs can vary widely, but replacing a thermostat heater typically ranges from $150 to $400, including parts and labor. Wiring repairs or ECM replacements may cost more.

9. Can a weak battery cause the P0599 code on a GMC?

While unlikely, a weak or failing battery can sometimes create electrical issues that may contribute to codes like P0599. It’s worth testing the battery and charging system during diagnosis.

10. Will clearing the P0599 code with a scanner fix the issue on my GMC?

Clearing the code may temporarily turn off the check engine light, but if the underlying issue isn’t resolved, the code will likely return. Proper diagnosis and repair are necessary to fix the problem.

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