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PO128 means the engine hasn't reached operating temperature within a predetermined amount of time. A stuck open thermostat is a possibility and a good place start especially since a coolant change was on your list.
Other causes could be low coolant level, so make sure it's up to where it should be, or a bad coolant temp sensor or related circuit/harness issues. You won't damage the engine unless it overheats, which isn't as likely if it's an open stat in this cooler weather. Keep an eye on the temp gauge.