Powertrain, generic · Vehicle speed, idle control and auxiliary inputs

P0599 Thermostat Heater Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

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

Symptoms

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€120–600
Time1–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

map controlled thermostatthermostat heaterheater wiringengine control module

Related codes

P0597P0598P0128P2181P0217

References: gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov, thegrouptrainingacademy.com.