P0128 Code: Meaning, Causes, and Symptoms | OBD2 Codes

P0128 – Coolant Thermostat Malfunction


P0128: DIY Repairing Video Tutorial


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P0128 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

P0128 Possible Symptoms

Engine Light ON (or Service Engine Soon Warning Light)


The thermostat is a component that blocks the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. Once the engine reaches its operating temperature, the thermostat opens. The OBDII sets when the engine coolant temperature has not risen enough to open the thermostat even though the engine has run long enough. This is due to a leak in the seal or the thermostat open stuck.