How to get at the heater core in a 2002 sentra?

If your coolant isn't low, it could be a stuck thermostat or a plugged heater core

yes they can do that

the may have a mouse nest in the air box. I have seen this same problem before. you'll need to remove the glove box and then the four or six screws ( can't remember witch) from the air box and you'll be able to see the doors, heater core and evaporator. the nest will probably be all over in there.

never post a car with no year, they make changes every year.
1990 to 2002 , ? and is a Toy Corolla in Snake skins(geo)
geo was 90-97/ only. 1.6L or 1.8L?
like heater water valves, or heater damper only heat controls.
ok.
so the fan runs at full speed?
the engine gets to 180F and holds?
ok not knowing year, ever see the 2 hose from engine side, to
cab side heater core. the that pass thru? well that is the cab core.
and you get engine hot and touch both hoses. are they both hot
yes, good, if not we work that.
this take like 1 min to do , after engine is hot and works on all cars.
1996 major changes. so?

here is a no year, answer on the controls
when you move each lever, it move a cable (does it) and if does
does the end of each cable move an object.
1: a water valve.
2: a blend door.

the cable has 2 end, either end can fail.
the cable has sheath, just like a bicycle and works the same way.
the sheath is has clamps if broke the cable is rendered useless.
use tie warp to clamp this floating sheath to any near object.
done many.
on old cars, i back flush all coolant paths and test the water valve if found. then when all together I have full flow through the heater core.
one guy had mouse nest in core, blocking it.
the system is real simple
hot water must flow thru the heater core. at 180F or more.
and air must flow through the out side of this core and past
all the way through it, and arrive at the vents
if the core is hot, then the flow is bad.
bad damper ,mouse nets. or some pack with leaves.
you now have all you need to fix it.

read this, they mostly bounce off walls but find it.
http://www.toyotanation.com/forum/134-6th-generation-1988-1992/373126-no-heat-1991-corolla-2.html

Do you get steam out of the exhaust? Foamy Oil on your oil dip stick? Oil slicks in your water? Overheating? These are all things that could point to a bad gasket. If you smell anti freeze or are getting steam out of your defrost .. that would point to a bad heater core. I base my answer on the assumption when you say no heat you mean your heater is not putting out hot air???

It should be around $50.00 plus ther cost of the alternator , Which will have a core charge until you turn in the old alternator and the core payment back.

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

does the fan motor come on when you turn the dial if so then you might not have a mouse as far as the heat goes it could be 2 things the thermostat or the heater core to check the heater core start the car let it get to normal temp and tuch the hoses that runs to it if they are hot then its good as for the thermastat the best way to do it is just replace it it might cost you ten bucks and 20 mins of time

You could have a head gasket problem that will destroy your cat and 02 sensors. BUT without spending much cash reverse flush your heater core internally first. If this doesn't work I've seen little friends make nests inside the blower motor housing. Take out glove box then drop down blower housing. Vacuum and blow out housing and reach in and clean externally the heater core. Both jobs will take inexperienced folk 4 hours.If this doesn't fix the be prepared to open up your wallet.

I had this same problem in my 1985 Chevrolet Citation X-11 before i took the Squirrell Cage (blower motor) out and shop-vacc'd the mouse nests out of it. there is probably something obstructing yours as well, my heater core was attached to the firewall in the engine bay but since your car is newer than the 80's i would say your heater core and blower motor are accessable underneath your dash. when you take the lower cover off the heater core be prepaired with rubber gloves and a shop vac just in case your nest falls out, you really dont feel like catching whatever those nasty critters are carrying. if you open up your heater core and move all the doors (indicated by foam sealing strips) and everything is empty and the doors move freely (dont push the doors, use the dash controls to operate them, usually by cycling slowly from hot to cold)  then that means you dont have a nest, but i would reccomend replacing your blower motor. Keep me updated!
Hope this helps!
                       Joe.