P0599: Thermostat Heater Control Circuit High

A map-controlled thermostat is a wax thermostat with a built-in heating resistor (Valeo: about 10 Ω). Unheated it opens late, for hot running at part load; the engine module heats it by map under full load or towing so it opens earlier. CARB requires the thermostat to be monitored and the heater circuit counts as an output component. GM switches it to ground with a low-side driver and checks it every second, fault after 15 of 30 samples, type B; Hyundai/Kia/Genesis and VW/Audi use the codes as well. P0599 means a short to positive or a shorted heating resistor – GM detects it with the driver on, when it cannot pull the line to ground. The heater has no effect, coolant runs hotter under load; Audi's 4.2 FSI reports it with P2181, shows the coolant warning and limits engine speed.

Symptoms

• Check engine light on
• Higher coolant temperature under load
• Coolant warning and engine speed limit (Audi)

Most likely causes

• Heating resistor in the thermostat shorted
• Control line shorted to positive
• Coolant or corrosion in the connector
• Engine module driver

What to check, in order

Start with the simplest. You can tick off each step.

1. With the engine cold, check the coolant level; never do this when the system is hot
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Check the actual temperature in live data
4. Make sure the fan comes on when it should
5. Check the thermostat and connector
6. Measure the sensor and replace it if its readings do not make sense

Good to know

• Repair difficulty: medium
• It can trigger limp mode.
• It affects emissions.

What each character of P0599 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 5: Speed and idle
• 99: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0599?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P0599?

The most common causes are: heating resistor in the thermostat shorted; control line shorted to positive; coolant or corrosion in the connector.

Related codes

• P0597Thermostat Heater Control Circuit/Open
• P0598Thermostat Heater Control Circuit Low
• P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
• P2181Cooling System Performance
• P0217Engine Coolant Over Temperature Condition

Sources

• gsi.ext.gm.com: 18OBDG03A Engine Diagnostics
• static.nhtsa.gov: MC 10128349 9999
• static.nhtsa.gov: MC 10142246 9999
• thegrouptrainingacademy.com: TSB VSA VEC 072023 01 Electric Thermostat Troubles

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.