Possible causes

– Leaking or stuck open thermostat
– Low engine coolant level
– Insufficient warm up time
– Faulty engine coolant temperature sensor
– Engine coolant temperature sensor harness is open or shorted
– Engine coolant temperature sensor circuit poor electrical connection

Tech notes

If cooling system is not leaking and the fluid level is OK, replacing the thermostat usually takes care of the problem.

When is the code detected?

The engine coolant temperature does not reach to specified temperature even though the engine has run long enough.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0128 HYUNDAI Description

Coolant thermostat (coolant temperature below thermostat regulating temperature). Indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.

HYUNDAI Technical Service Bulletins for P0128 code