Code Definition

P0599 — Thermostat Heater Control Circuit High

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P0599

P0599 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0599 is stored, the module has detected a signal or supply voltage higher than the expected operating range.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0599 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The Engine Control Module (ECM) controls the electrically heated thermostat to regulate coolant temperature based on engine operating conditions, and monitors the heater control circuit for correct voltage behaviour. DTC P0599 is set when the ECM detects a higher than expected voltage in the thermostat heater control circuit when the heater is commanded on or off. The heated thermostat uses an internal heating element that the ECM energises to open the thermostat earlier than the wax element alone would, allowing tighter control of coolant temperature for emissions and fuel economy. A high vol...

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