P0599Thermostat Heater Control Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Check engine light on
- Higher coolant temperature under load
- Coolant warning and engine speed limit (Audi)
- Likely causes
- LikelyHeating resistor in the thermostat shorted
- PossibleControl line shorted to positive
- PossibleCoolant or corrosion in the connector
- RareEngine module driver
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €120 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.