The car drives normally but uses more fuel, and the heater will be disappointing in cold weather. A thermostat is an inexpensive part and this code usually means exactly what it says.
The engine has run long enough and worked hard enough that it should have reached thermostat temperature, and it has not. This is about as specific as generic diagnostics get: the module has effectively concluded the thermostat is stuck open. It is one of the few codes where replacing the named part first is genuinely the right call, though checking coolant level costs nothing and occasionally explains it.
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P0128 means the engine never reached the coolant temperature the thermostat is supposed to hold. The ECU waited, did the maths against ambient temperature and running time, and con...
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