Coolant Thermostat Below Regulating Temperature
Powertrain Code · Medium
What does P0128 mean?
The engine coolant is not reaching the expected temperature. Most commonly caused by a stuck-open thermostat, low coolant level, or a faulty coolant temperature sensor.
System: Coolant System
P0128 is a powertrain diagnostic trouble code indicating coolant thermostat below regulating temperature. This code is classified as medium severity, which means: schedule service — affects performance or emissions. We have vehicle-specific diagnostic data for 50 models below. Select your vehicle for tailored causes, symptoms, and repair cost estimates.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your vehicle.
See causes, symptoms, and repair costs for P0128 on specific vehicles: