P0599 moderate

Thermostat Heater Control Circuit High

The P0599 code means the vehicle's onboard diagnostic system has detected: Thermostat Heater Control Circuit High. This is a moderate severity code.

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Keep driving?: Yes, but fix soon
DIY difficulty: moderate
Estimated cost: $100 - $400 (parts and labor)

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Common Symptoms

Check Engine Light on
Heated thermostat circuit reads high
Engine slow to reach operating temperature
Poor fuel economy during warmup

Probable Causes (Ranked by Likelihood)

Open or short to power in thermostat heater circuit Very High
Wiring damage or corrosion in the circuit High
Poor connector contact or backed-out pins Moderate
ECM or control module circuit failure Low

Step-by-Step Diagnostic Procedure

Use a scan tool to read Thermostat Heater Control related data
Inspect all related wiring and connectors for damage, corrosion, or loose connections
Measure circuit voltage and resistance to identify opens, shorts, or high resistance
Command the affected component with a scan tool to verify it responds to ECM commands

Frequently Asked Questions

What is an electrically heated thermostat?

Some modern vehicles use an electrically heated thermostat that can be commanded open by the ECM before coolant temperature alone would cause it to open. This allows faster warm-up of certain engine components and more precise temperature management.

Will P0599 cause overheating?

Typically no. If the heated thermostat circuit fails, the thermostat still operates as a conventional thermostat using its wax element. The engine may take slightly longer to warm up and fuel economy during warmup may decrease slightly.

Are heated thermostats expensive?

Heated thermostats cost more than conventional thermostats, typically $50 to $150 for the part. The thermostat itself usually needs replacement since the heating element is integrated into the thermostat assembly.