Thermostat Heater Control Circuit High
The P0599 code indicates that the powertrain control module (PCM) has detected a high voltage condition in the thermostat heater control circuit. This circuit is responsible for regulating the engine coolant temperature by controlling the electric heater element within the thermostat. A high voltage reading suggests a short to power, faulty heater element, or wiring issue, which can prevent the thermostat from opening properly, leading to engine overheating or poor fuel economy.
01. Most Common Causes
- Shorted thermostat heater element
- Short to power in the heater control circuit wiring
- Faulty PCM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine overheating or running too cool
- Poor fuel economy
03. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for related codes.
Step 2: Visually inspect the thermostat heater connector and wiring for damage or corrosion.
Step 3: Test the heater circuit voltage with a multimeter; compare to manufacturer specs.
Step 4: If voltage is high, isolate the circuit to find the short; replace the thermostat if heater element is faulty.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended as it can cause engine overheating or poor performance. Have it diagnosed promptly.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on the cause and vehicle.