What Does P0599 Mean?
The thermostat heater control circuit is reporting a high voltage condition. The PCM detects elevated voltage on the heater control line, typically indicating an open circuit in the ground path, a disconnected thermostat heater, or a short to a voltage source. The heated thermostat feature will not function, but the thermostat will continue to operate mechanically using its wax element.
Common Causes
30%
Thermostat heater element open circuit (burned out heating coil)
25%
Open or broken wire in the thermostat heater control circuit
20%
Disconnected or corroded thermostat heater connector
15%
Short to battery voltage in the control wire
10%
PCM output driver open or not commanding the ground signal
Diagnostic Steps
1
Verify the thermostat heater connector is plugged in and secure — check that coolant leaks haven't pushed the connector off or corroded the terminals.
2
Measure the heater element resistance at the thermostat connector — typical is 10-30 ohms; OL (open loop) confirms a burned-out heater element requiring thermostat replacement.
3
With the connector unplugged, verify B+ is present at the power supply pin and that the PCM is providing a ground signal on the control pin when conditions are met (cold engine).
4
Check for voltage on the control wire with the connector unplugged — B+ voltage when the PCM should be grounding the circuit indicates either a wiring short to voltage or a PCM driver fault.
5
If the thermostat is relatively new but the heater has failed, check for coolant intrusion into the heater element area — a leaking thermostat housing gasket can allow coolant to contact the heater coil and cause premature failure.
Estimated Repair Cost
$150 - $500
Parts + labor, varies by vehicle and location
The thermostat heater control circuit is reporting a high voltage condition. The PCM detects elevated voltage on the heater control line, typically indicating an open circuit in the ground path, a disconnected thermostat heater, or a short to a voltage source. The heated thermostat feature will not ...
The most common cause of P0599 (Thermostat Heater Control Circuit High) is: Thermostat heater element open circuit (burned out heating coil)
Typical repair costs for P0599 range from $150 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to drive. The thermostat will operate normally using its mechanical wax element at a fixed opening temperature. The only downside is slightly reduced fuel economy and less precise engine temperature management. There is no risk of overheating, as the mechanical thermostat still opens and closes normally.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0599 to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Cooling System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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