P0598: Thermostat Heater Control Circuit Low

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.

Symptoms

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

Most likely causes

• Internal short in thermostat or heater, coolant in the connector
• Control line shorted to ground (chafing)
• 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. With the connector unplugged, check whether the signal wire is making contact with ground.
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 affects emissions.

What each character of P0598 means

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

Frequently asked questions

Can I keep driving with code P0598?

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 P0598?

The most common causes are: internal short in thermostat or heater, coolant in the connector; control line shorted to ground (chafing); engine module driver.

Related codes

• P0597Thermostat Heater Control Circuit/Open
• P0599Thermostat Heater Control Circuit High
• P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
• P0125Insufficient Coolant Temperature for Closed Loop Fuel Control

Sources

• gsi.ext.gm.com: 18OBDG03A Engine Diagnostics
• static.nhtsa.gov: MC 10142246 9999
• static.nhtsa.gov: MC 10142245 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.