What does po128 code mean on the chevy malibu?

The Chevy Malibu P0128 code is letting You know that Your coolant thermostat temperature is below the regulating temperature for optimal efficiency. There could be many causes for this such as a faulty or stuck thermostat, bad coolant temp sensor, and a radiator fan that does not turn off. The likely issue is the thermostat and is what most mechanics would replace if You get this error code. The thermostat is very cheap and can be accessed easily so it is a quick fix that will usually address the problem. When You replace the thermostat also make sure to flush the engine block and radiator to prevent particles and debris from sticking to the new thermostat.