2004 Honda Civic. How to remove heater core please?

remove the hoses to the firewall and splice in a pipe and clamp it off.

the heater core is connected to the engine cooling system. use radiator flush and run the engine to normal operating temperature with the heat on. follow the directions on the flush product.

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Get a repair manual and don't even go near the
airbags without removing the negative battery
cable.

flush the heater core with a garden hose ,and dont forget to top up with antifreeze

if this only occurs only when you wash the car or it rains then you either have a rust through at the cowl or front pillar or the seal around the windshield is leaking. sounds like a job for the glass shop. they will remove the glass check for rust and replace the seal.

Check your coolent level it could be that your heater core is leaking inside your dash. if its low then have your heater core checked

It is probably low level. Check it and fill it to proper level. If low, it won't circulate through the heater core, when you accelerate, the water pump speeds up, some coolant now makes it to the heater core. Watch the level to make sure it doesn't drop again, you may have a leak somewhere.

u need to open the blower motor assembly and clean all the old dry leaf that build up & blocking the heater core
the fan (blower motor) is unable to pull the air tru

first check to see if the hoses to the heater core are hot. if they are not hot do the following.
Check the valve where the heater core hoses go into the cab, if the valve is good , you may have to wash out the heater core by removing both hoses at the firewall and run some water through the heater core until the water comes out clean of rust and debris.