P0599 code meaning
Thermostat Heater Control Circuit High
Powertrain system Generic code P05xx codes Check engine light
What P0599 means
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.
P0599 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium, DIY possible
- Typical labor1 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P0599
- Check engine light on
- Higher coolant temperature under load
- Coolant warning and engine speed limit (Audi)
Common causes of P0599
- Most likelyHeating resistor in the thermostat shorted
- PossibleControl line shorted to positive
- PossibleCoolant or corrosion in the connector
- Less likelyEngine module driver
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
MAP controlled thermostatThermostat heaterHeater wiringEngine control module
What each part of P0599 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 5 | Area: vehicle speed, idle control and auxiliary inputs. |
| 99 | The exact fault in that area. |
What to do next
- Note P0599 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0599
What does P0599 mean?
P0599 means "Thermostat Heater Control Circuit High". A map-controlled thermostat is a wax thermostat with a built-in heating resistor (Valeo: about 10 Ω). It is a generic code, so it means the same on every make.
Is it safe to drive with P0599?
Drive gently and only to a workshop. With P0599 the car can go into limp mode (reduced power) to protect itself.
Will P0599 fail an emissions test?
Yes, it can. P0599 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0599 to fix?
Difficulty: medium. A typical repair takes 1 to 3 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0599 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.