OBD-II code · generic

P0128

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

System: Cooling system (thermostat / coolant temperature sensor)
Severity: Moderate (3/5)
Can I keep driving?: Drive with caution

What does P0128 mean?

P0128 is defined as “Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)”. This code indicates a problem with the thermostat, the engine coolant temperature sensor or coolant flow. The engine may take too long to reach operating temperature, or overheat.

Drive gently and only as far as you need to; have it checked as soon as possible.

Symptoms

Temperature gauge stays lower than normal or fluctuates
Heater warms up slowly, higher fuel consumption
Overheating (if the thermostat is stuck closed) and the check engine light

Common causes

Thermostat stuck open (the most common cause of “low temperature” codes)
Fault in the heater circuit of an electronic thermostat
Faulty coolant temperature sensor or connector
Low coolant level, air in the system

How to diagnose it

Check the coolant level with the engine cold.
In live data, watch the temperature rise steadily as the engine warms up and settle around 85–105 °C.
If the top radiator hose warms up before the engine does, the thermostat is stuck open.
Measure the sensor resistance and the electronic thermostat heater resistance.

How to fix it

Replace the faulty thermostat together with its gasket.
Replace the sensor if it is faulty.
Top up the coolant and bleed the air from the system; clear the code and confirm the temperature settles normally.

Manufacturer-specific meanings

The same code can mean something different on some makes.

Kia: Coolant Thermostat Malfunction

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This page is general information; an exact diagnosis needs measurements and a technician’s check.