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

P0598 Thermostat Heater Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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. P0598 means the control line is shorted to ground – GM detects 0.5 Ω with the driver off. With a low-side driver the heater then stays on permanently and the thermostat opens early, so the engine runs cool; Kia's Cadenza bulletins list it with P0128 and replace the thermostat.

Likely causes

Internal short in thermostat or heater, coolant in the connector
Control line shorted to ground (chafing)
Engine module driver

Symptoms

  • Check engine light on
  • Engine slow to reach operating temperature (possibly P0128)
  • Weak cabin heating in winter, higher consumption

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

P0597P0599P0128P0125

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