Could be a head gasket or a warped or cracked Cylinder head.
Even though you have already replaced the thermostat, there are other issues that can cause your Nissan Maxima to overheat. Your fan may be stuck and not operating as it should.
Does that mean that you replaced the thermostat AND the sensor? If not, you really need to replace the thermostat. A sticking thermostat can cause temperature swings and potentially overheat the engine. BTW, BMW engines tend to have serious problems when they overheat. Don't let it overheat or you might be buying a new engine.
water pump
The thermostat is most likely stuck in the closed position and needs to be replaced. The closed thermostat does not allow the coolant to circulate through the radiator, thus overheating the engine. The reason for water being pushed through the thermostat housing is because the water pump is spinning and there is no place for the coolant to go, therefore, it is leaking out through the housing gasket which is probably already blown (need to replace it when you replace the thermostat.
Would need to know if car was overheating before waterpump and thermostat was replaced.
It could be you thermostat Have you have already checked oil level, and/ot changed oil recently Exaust restrictions can cause overheating
Since you have replaced these two parts, have you checked the water pump ? With age, the impeller on the pump or bushings become used enough to reduce the flow of coolant quite a bit. This will cause over heating due to lack of circulation. Also did you flush the system to make sure that there are no blockages ? And, check the upper and lower hose to make sure they do not collapse.
it could be the water pump or a blown head gasket
Try flushing heater core Try flushing heater core
Could be several reasons but we really need to know the year, make and model of your vehicle to help you.
How did you change the thermostat?
flush out heater hoses