P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

System: Engine
Dashboard light: Check engine light (MIL)
Check-engine blink code: —
Applies to: Generic OBD-II meaning. Not every code is used by this car.
Description: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Detection / symptoms: 1.Check engine light on 2.You will likely not notice any drivability problems other than the 3.Illumination. In some cases you may experience symptoms such as 4.Engine takes longer to warm up 5.Engine temperature drops while driving at higher speeds 6.A photo of a typical thermostat
Possible causes: 1.Low engine coolant level 2.Leaking or stuck open thermostat (likely) 3.Faulty cooling fan (running too much) 4.Faulty 5.Coolant temperature (ECT) sensor 6.Faulty intake air temperature (IAT) sensor
What to check: 1.Read the code and freeze-frame data with an OBD-II scanner 2.Check the connector and wiring of the part named in the code 3.Test the part with scanner live data or a multimeter, using the values in the repair manual 4.Fix the cause, clear the code and test-drive
Confidence: medium (generic OBD-II definition, not checked against the factory manual for this car)

Generic OBD-II meaning (obd-codes.com), not checked against this car's factory manual. Clear the code and test-drive to confirm it returns. Search all fault codes · Common problems · Warning lights