I have a 2002 Nissan Sentra the thermostat opens, changed water pump. the heater still blows cold air .

yeah, low on coolant, or a blockage in the cooling system. I would do the thermostat first, should be simple and fairly inexpensive.

If you need further help, reach me via phone at https://www.6ya.com/expert/tom_1788908372a4836f

remove both hoses from engine that passes through firewall to heater. put a stiff piece of wire in the hole and make sure it is clear. If there is no restriction, then the heater is choked up

Might try a reverse flush of the cooling system. Might reverse flush the heater core too.

The problem is likely in your heater case doors, not the core itself. These doors are either vacuum operated or small motor operated.

Get the car warmed up, then feel both heater hoses. Both should be hot, meaning coolant is circulating. If only one is hot, the other cold, then you have a circulation problem.

Is it possible that you have a temp door problem in the dash. Maybe the cable from the temp control came off the arm for the door.

check the heating core or radiator for clogging or leaks

you could have an air pocket in the cooling system that has to be burped. With a cold engine, park uphill and leave the radiator cap loose. Fill radiator and coolant reservoir. Start engine with cap loose and run at 1500 rpm until temp gauge reads normal. Feel if top radiator return hose from engine to radiator is hot. If yes, coolant is flowing and thermostat is open. If cold, thermostat is not staying closed until engine is hot.

report results.

My 2005 Nissan Sentra 1.8s with 55,000 mi. blows cold air from the heater. Coolant not circulating in engine. Lower hose stays cool to touch. Both heater hoses cold. Changed thermostat - didn't fix it. Coolant not leaking or dirty. None in oil - no oil in radiator. Changed worn water pump belt - pump spins freely with no noise or resistance. Cooling fans come on after warm-up. Temperature gauge stays in normal range then jumps to hot and back again. Coolant good to 10 below - didn't freeze here yet. No other problems or repairs to car. Afraid to drive and make worse. Help!

My 2005 Nissan Sentra 1.8s with 55,000 mi. blows cold air from the heater. Coolant not circulating in engine. Lower hose stays cool to touch. Both heater hoses cold. Changed thermostat - didn't fix it. Coolant not leaking or dirty. None in oil - no oil in radiator. Changed worn water pump belt - pump spins freely with no noise or resistance. Cooling fans come on after warm-up. Temperature gauge stays in normal range then jumps to hot and back again. Coolant good to 10 below - didn't freeze here yet. No other problems or repairs to car. Afraid to drive and make worse. Help!

You will have to replace the thermostat...It sounds like the thermostat is not opening and closing all the way. When the car is running fast enough to heat up the water, great...heat...but at stops the thermostat is not closing all the way allowing the water to cool as it passes through the heater.