Engine code P0128

[1967mustang]

Can anyone tell me what this code means? I have a 2003 Nissan Maxima. 3.5 motor

[ezb57e]

It means the coolant temp has not reached the level it should have in the amount of time the engine has been running.
Typically, this a stuck open thermostat, but there has been ECM software changes on some other vehicles. Off the top of my head I don't think your car is one of them.
Make sure its full of coolant for starters, or that the radiator fan isn't running constantly.