Sorry to say, and I know it doesn't make much sense, but you most likely have a blown headgasket. I'm in the midst of replacing the headgasket myself on my 2002 Sentra. Not sure why, but it seems this is not an uncommon issue with these cars. Exhaust gas is getting into the coolant galleys within the head, which creates an airbubble of sorts that creates the overheating problem. Don't drive it around too much or you risk warping the head. Take it to a mechanic. I thought it was a thermostat myself, hoping against hope. The cold air from the heater is a result of poor coolant circulation through the heater core due to the void created by the rupture in the headgasket.
The Nissan Sentra thermostat diagram is available from the Nissan service center. This will be provided to you upon request.
The thermostat on a 1995 Nissan Sentra 1.6 is located behind the water inlet. The water inlet is positioned at the end of the lower radiator hose.
The thermostat on a 1989 Nissan Sentra is on the front of the motor to the left. It is just below where the valve cover meets top of the manifold. The thermostat is enclosed in the radiator hose at that location.
The thermostat on a 1995 Nissan Sentra 1.6 is located behind the water inlet. The water inlet is positioned at the end of the lower radiator hose.
follow the top hose off of your radiator and this is where your thermostat is
Follow the bottom hose to the thermostat housing on the front side of the engine.
Our 2003 Nissan Sentra themostat lasted 82,000 miles, getting it back today...big$$$ HRMOKeefe@aol.com If you follow the top radiator hose back to the top of the engine, that is the thermostat housing.
If your security sensor in your 2001 Nissan Sentra 2.0 is jarred or jammed you should have it replaced.
Look for the thermostat housing on the back side of the block on the upper left-hand side.
maybe check the thermostat.
Look for a removable housing at the engine end of the upper radiator hose
Normally it will last the life of the engine and is replaced when the engine is overhauled.