Coolant leaking from heater area, by fire wall. Is heater core bad? Where is it and is it hard to replace.?

undo the both heater hoses at the engine fire wall
turn the heater control on
apply a garden hose at one of the hoses and flush through
do it both ways
Two things to consider
stop leak should only work at the area of the leak and circulate normally with the coolant so there should not be a blockage
second point is , if the heater core is blocked then flushing will hot clear it and it may need a new core
It is known that some stop leaks affect the coolant control tap of the heater so it may be that control unit that is the problem and not the core so keep that in mind before replacing the core

You do not list what kind of vehicle you have so it is very hard to explain anything to you. That said, all heater cores that I have replaced have been done by removing it through under the dash.

the heater core is bad in summer u just route your heater hose's togther where they go thru the fire-wall.put a splice between both hoseses so water wont go into heater-core in winter u need heater u must if u have to use vehicle slow-speed only remove rariator cap pressure then leak wont leak as fast u can use heater until fluid is all leaked out or u can just move heater slide to blue area then water wont be going to heater core it wont leak out as fast or replace heater core

no it wouldn't be the thermostat sounds like the evap for the a/c or the heater core it bad both are on the fire wall the a/c evap is in the middle of the fire wall the heater core should be on pass side fire wall

Do you have coolant on the front passenger floor board? When heater cores fail it will leak onto the floor board and mist coolant on the windshield. If you have a loose clamp on the heater core hose it's possible it could be leaking down the firewall into the passenger compartment. If you don't have coolant on the windshield check the clamps / hoses first.

That drain line is for the box that holds the air conditioner evaporator and the heater coil. It seems that your heater coil has sprung a leak and the leaked fluid is coming out the drain. You have to replace the heater coil, which on most cars involves removing a lot of the interior equipment, including the dash board on many cars. A job best left for the repair shop unless you're very adventurous and patient. Even then you will have to take it to a shop to have the air conditioning system recharged.

Is the water coming out of the a/c drain or is it one of the heater hoses going into the fire wall. If its coming out the a/c drain you need a new heater core. if its outside its just a rotten hose.

Hi. the heater core is leaking. i would replace This damaged core asap. there is a shut-off valve you can use to heel the leak until you can repair this issue. the shut-off valve is located under the hood on the fire wall center area.

those are 2 hoses an inlet and outlet for your heater.take a look inside the car at the carpet and see if there is anti freeze.the heater core can leak,and then run down the hose,but you may be lucky and just the hose is no good.they are quite stiff to take off,you have to twist them as well as pull.go slowly,damage to the heater core can result otherwise.the heater core is mounted on the firewall and the entire dash must be removed to replace it.if you do cahnge it,make sure there is a ground strap added from the heater pipes to ground.it prevents electrolysis from the dissimilar metals.

   Yes it is the heater core. Major job to tackle.