Thermostat Heater Control Circuit High
P0599 indicates the engine control module detected a high voltage condition in the thermostat heater control circuit. This electronic control system regulates coolant temperature by operating an electrically heated thermostat, and a high voltage signal means the circuit is not functioning as designed.
What You Might Notice
- Check engine light illuminated
- Temperature gauge reading abnormally high or fluctuating
- Possible rough idle or hesitation during startup
- Reduced fuel economy
- No obvious drivability issues in some cases
Most Common Causes
- 1
Loose or corroded electrical connector
A corroded or loosely connected electrical connector at the thermostat heater causes high resistance and voltage reading errors. This is the most common cause and easily corrected.
- 2
Thermostat heater element failure
The electronic thermostat heating element may be stuck open or shorted, preventing proper circuit voltage control. The thermostat assembly would need replacement.
- 3
Wiring short or open circuit
A short to power or open condition in the thermostat control wiring harness causes abnormal voltage signals to reach the engine control module.
- 4
Engine control module failure
The Motronic/ECM circuits responsible for monitoring the thermostat control voltage have failed. This is least likely but possible after other causes are ruled out.
How to Diagnose It
- 1
Check Loose or corroded electrical connector — A corroded or loosely connected electrical connector at the thermostat heater causes high resistance and voltage reading errors. This is the most common cause and easily corrected.
- 2
Check Thermostat heater element failure — The electronic thermostat heating element may be stuck open or shorted, preventing proper circuit voltage control. The thermostat assembly would need replacement.
- 3
Check Wiring short or open circuit — A short to power or open condition in the thermostat control wiring harness causes abnormal voltage signals to reach the engine control module.
- 4
Check Engine control module failure — The Motronic/ECM circuits responsible for monitoring the thermostat control voltage have failed. This is least likely but possible after other causes are ruled out.
How to Fix It
- 1
Inspect and clean connectors
Locate the thermostat heater electrical connector and inspect for corrosion, loose pins, or green/white oxidation. Clean with electrical contact cleaner and dielectric grease, then reseat firmly. Clear the code and test drive.
- 2
Check wiring for damage
Visually inspect the thermostat control wiring harness from connector to ECM for cuts, chafing, or burnt insulation. Look for signs of shorts or open circuits. Repair or replace damaged sections of wiring.
- 3
Replace thermostat assembly
If connectors and wiring are good, the electronic thermostat heater element is likely defective. Remove the old thermostat housing and install a new thermostat assembly, then refill coolant and clear codes.
- 4
Test or replace engine control module
If all above steps fail, the ECM may have a faulty thermostat control circuit. Have the module tested by a specialist or replace it as a last resort. This requires reprogramming and is uncommon.
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