Heater keeps blowing out cold?

Hello,

Blowing cool out for the heater can be due to few different items.
It could be as simple as being low on coolant, can be a stuck open thermostat, a faulty blend door actuator or a clogged heater core.

If the thermostat fails at stuck open position it will now allow coolant to reach the operating temperature. When this happens the heater will blow cool air instead of warm/hot.
Check to see if the engine reaches the operating temperature. If the gauge shows cool then you likely have a stuck open thermostat.

https://www.2carpros.com/articles/how-an-engine-thermostat-works
https://www.2carpros.com/articles/replace-thermostat

A blend door actuator controls the flow of hot or cold air. it controls a 'door' that changes with the setting you select.
A faulty blend door actuator can fail to either hot or cold side. This can make the vehicle only blow either hot or cold.
The blend door actuator is located under the dash on the passenger side.

https://www.2carpros.com/articles/replace-blend-door-motor

Also you could have a restriction or clog in the heater core. If the coolant does not flow thru the heater core you will not have warm air for the heater.

https://www.2carpros.com/articles/replace-heater-core

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.