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The code often shows up when the coolant temperature sensors are reading incorrectly or the engine control module isn't responding properly - it's worth testing the sensors before you go ahead and replace the thermostat.
I've run into this code myself - P0128 means your engine isn't reaching or maintaining the correct operating temperature. Usually it's a faulty thermostat or coolant issues. Get it checked out pretty soon, because otherwise your engine runs inefficiently and can take damage over time.
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