Faulty thermostat heater element (internal short or open circuit) in the electronic thermostat; Damaged, chafed or corroded wiring to the thermostat heater, including a short to voltage; Corroded, loose or water-contaminated connector at the thermostat; Fault in the control circuit's driver inside the engine control unit; Relay or fuse supply fault feeding the thermostat heater circuit, on models that use one; Outdated control unit software where the manufacturer has issued a known fix.

P0599

Thermostat Heater Control Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
99: FaultThe specific fault in that area

What it means

P0599 is the generic SAE J2012 code for a thermostat heater control circuit high fault. The engine control unit drives a heating element in an electronically controlled (map-controlled) thermostat so it can open earlier and run the engine cooler under load. The code sets when the voltage or state it sees on that control circuit is higher than it expects for the command it is giving.

Symptoms

Engine warning light on
Engine running hotter than normal under light load or cooler than normal on the motorway
Slow warm-up or poor cabin heat
Temperature gauge reading higher or lower than usual
Slightly worse fuel economy
No other noticeable symptoms in some cases

Possible causes

Faulty thermostat heater element (internal short or open circuit) in the electronic thermostatlikely
Damaged, chafed or corroded wiring to the thermostat heater, including a short to voltagelikely
Corroded, loose or water-contaminated connector at the thermostatlikely
Fault in the control circuit's driver inside the engine control unitrare
Relay or fuse supply fault feeding the thermostat heater circuit, on models that use onepossible
Outdated control unit software where the manufacturer has issued a known fixrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including coolant temperature and engine load when the code set
Check for other cooling system or coolant temperature sensor codes, which could point to a related fault
Visually inspect the thermostat housing connector and wiring for heat damage, chafing, corrosion, coolant contamination or pushed-out pins
Check coolant level and condition, and confirm the engine warms up and holds a sensible temperature
Use a scan tool with live data or actuator tests, where supported, to see if the control unit commands the heater and whether the circuit responds
With the connector unplugged, measure the heater element resistance and compare it with the manufacturer's specification for that engine
Check the supply voltage, earth or control signal and wiring continuity, including a check for a short to voltage, using the wiring diagram for the model
Check for relevant software updates or technical bulletins for the specific model and engine

How to fix

Replace the electronic thermostat or its heater element if the resistance is out of specification
Repair or replace damaged wiring and fix any short to voltage in the heater circuit
Clean, repair or replace the corroded or damaged connector and its terminals
Replace a faulty relay or repair the fuse supply where the circuit uses one
Replace or repair the engine control unit only after the wiring and thermostat are proven good and the driver fault is confirmed
Update the control unit software only if a manufacturer fix applies to this fault

Can I keep driving?

Medium severity

The car can usually be driven short-term, but the engine may not be regulating its temperature properly, so get it checked soon. Stop and seek help if the temperature gauge rises into the red or you see signs of overheating.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.