Quick checklist
Check coolant temperature with scanner verify thermostat operation
OBD2 Codes
P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Engine not reaching proper operating temperature; thermostat likely stuck open.
Details
The engine coolant is not reaching the proper operating temperature. The thermostat may be stuck open.
Typical causesStuck open thermostat faulty coolant temperature sensor
SymptomsCheck engine light poor heater performance poor fuel economy
SeverityLow — mainly affects fuel economy and heater performance
DIY checksCheck coolant temperature with scanner verify thermostat operation
ToolsTemperature scanner OBD2 scanner
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FAQ
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Can I drive with this code?
Yes but you'll have poor fuel economy and weak heater performance.
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