Code dictionary Generic OBD-II
P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
This is a definition, not a repair instruction. It does not identify a failed part or establish that a vehicle is safe to drive.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Separately researched
Vehicle-specific diagnosis
These guides address the exact vehicles and conditions in their sources. Select the guide that matches your vehicle.
Mercedes-Benz
Mercedes P012800 on M270/M274: check software and the thermostat test
Engine identity and a cold-start check determine the appropriate diagnosis.
Hyundai
Hyundai P0128: when the answer is a thermostat, and when it is software
Compare the 2017 Elantra thermostat campaign with the 2018 Elantra GT and 2019 Veloster 1.6T software campaign before authorizing a repair.
Ram
2019 Ram 1500 eTorque P0128: check the revision and all three modules
Check the later bulletin, companion faults and PCM/HCP/TCM completion before approving an eTorque repair.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT