P0599 indicates a high voltage condition in the control circuit of the thermostat heater. This heater is used in some vehicles to warm the coolant faster for improved emissions and cabin heat; the code means the powertrain control module (PCM) has detected a voltage above the expected range in that circuit.
A frayed wire, chafed insulation, or a pinched harness can cause the control circuit to see battery voltage when it should not. This is the most common cause.
The heater element inside the thermostat housing can short internally, creating a low-resistance path to power. This is less common than wiring issues but still plausible.
Moisture or corrosion at the thermostat heater connector can create intermittent high-resistance or direct shorts, triggering the high-voltage code.
A shorted relay or incorrect fuse can supply continuous power to the heater circuit, causing a high-voltage reading. Check the vehicle's specific circuit design.
A failed internal driver in the PCM can cause a false high-voltage reading. This is the least likely cause and should only be considered after thoroughly testing wiring and the heater element.
Yes, in most cases the vehicle will remain drivable. However, the thermostat heater may not function, which can increase warm-up time and potentially affect emissions. It is safe to drive short distances, but the underlying electrical issue should be diagnosed and repaired.
No. P0599 is an electrical circuit code, not a mechanical thermostat performance code. It indicates a high voltage condition in the heater control circuit, not that the thermostat itself is stuck. A separate code (e.g., P0128) would indicate a thermostat stuck open.
Almost never. The PCM is the least likely cause. Most P0599 codes are resolved by repairing a shorted wire, cleaning a corroded connector, or replacing the thermostat heater element. Only after all other possibilities are exhausted should the PCM be considered, and even then professional diagnosis is recommended.
The thermostat heater is an electric heating element built into the thermostat housing. It helps warm the engine coolant more quickly during cold starts, reducing emissions and improving cabin heater performance. It is controlled by the PCM and is not present on all vehicles.